Some fans might have been put off by how Optimus Prime killed Megatron in Dark of the Moon, because that is not how fans see Optimus. When you look over at Transformers: Prime, and see how Optimus was intent on killing Megatron after Raphael is nearly killed, the scene's brilliance comes into play. Because of Megatron's actions over the course of Revenge of the Fallen and Dark of the Moon, Megatron had gone too far in Optimus' eyes, just as his Transformers: Prime counterpart did.
On another side, considering that Optimus's master, Sentinel, teamed up with Megatron because he thought that the Autobots were never going to win, Optimus probably refused Megatron's truce because then he would be no better than his master.
Fans have noted Optimus' increasingly bloodthirsty behavior ever since Revenge of the Fallen, which doesn't just extend to enemies, but includes for example casually disposing of the parts of Jetfire which the latter donated at the cost of his own life. That's cold, at best. But one must remember that Optimus had just been resurrected after being violently killed by Megatron. He Came Back Wrong, whether it's "simply" PTSD or some kind of permanent damage to his spark that even the Matrix of Leadership couldn't heal.
The Fallen's death becomes this when you think about it. The Fallen was personally responsible for the entire war, a war that Optimus has seen the horrors of first hand. In that perspective, it's little surprise that Optimus went for No Kill like Overkill in dispatching the monster who was behind everything to happen to the Transformers for centuries.
Why does Optimus violently destroy anything/anyone who threatens Earth? Simple: He wants to make sure Earth doesn't wind up a barren wasteland like Cybertron, with its native inhabitiants reduced to near extinction.
Plus, when you consider how small Transformer heads are in proportion their bodies, it most likely takes more effort and focus to take out the head.
And as we saw, Frenzy continued to live in decapitated form, quickly taking another mode. If a Cybertronian doesn't destroy the head of his foe, he could easily find later on that one of his recovered weapons is secretly the disguised form of the foe's head, which could literally bite you in the ass later.
And there's also the fact that in every continuity, including this one, Transformers even when fighting against members of their own race, are extremely hard to take down permanently. Such as the fact that Megatron by the end of ROTF had not only lost his lower right arm but a good chunk of his head and managed to keep going. A full on decapitation is shown to be one of the few things that's immediately fatal even for them and then, as mentioned above that isn't always the case.
The Fallen was planning to betray Megatron. If the Solar Harvester went off, it either would've blown up the sun, which would most likely wipe out the Earth, or the entire planet would freeze really quickly. No wonder the Decepticons, who had several decades to search for it, never seemed to find it.
In the first Transformers film, Megatron went after the Allspark by himself, leading to him crash-landing in the Arctic. In DOTM Megatron states he and Sentinel Prime was supposed to rendezvous on Earth but fate waylaid them both. You realize that Sentinel Prime gave the location of the Allspark to Megatron. It was probably Sentinel Prime's Space Bridge that propelled the Allspark to Earth in the first place. The original plan was for Megatron to retrieve the Allspark while Sentinel Prime brings over Cybertron via the pillars. Together they would end the war and rebuild Cybertron.
The Fallen thought Optimus Prime is the last Prime as he felt his death. Optimus was not the last Prime, there is another, Sentinel Prime. The Fallen could not detect Sentinel as he was in stasis and thus assumed the latter died while trying to escape Cybertron in the Ark.
Megatron serves the Fallen yet it turns out Megatron has an alliance with Sentinel Prime. The Fallen wants every Prime dead as they can kill them. Thing is Megatron never intended to be second fiddle to the Fallen. In ROTF Megatron stated "Even in death, there is no command but mine." and in DOTM "All I want is to be back in charge". Megatron wanted to use Sentinel Prime to get the Autobots to surrender and use him to kill the Fallen to secure undisputed leadership of the Decepticons.
The Transformers are aliens capable of downloading and speaking Earth's languages. But remember they still have their own native tongue. Even among Earth languages there are words or concepts that do not translate very well from say, Japanese to English. If we can get these kinds of translation difficulties arising on the same planet, then it's almost guaranteed that the culture and language of an alien race is going to have words and concepts that are very hard, if not impossible to convey properly in the span of a few seconds. So when the ancient Primes told Sam that Optimus was their last "descendent" they probably didn't mean he comes from their lineage in whatever passes as Cybertronion biology. They probably just chose that word because, based on the 3rd definition here, it was the closest they could get to a literal translation.
Optimus' final words to Sentinel Prime before he kills Sentinel are this, when you consider their relationship and what happens afterwards. Sentinel Prime was the one who taught Optimus Prime all of the values that he holds in this continuity, and Optimus respected him to the extent he was willing to hand over the Matrix of Leadership to Sentinel when he was found. But Sentinel betrayed those values by siding with Megatron, and sees himself as superior to the Humans in every way, and tries to convince Optimus of this viewpoint, citing how the Humans see the Autobots as simple machines. So Optimus' words are a departure from the usual formula of vehement hate for traitors, and exist as a counter to Sentinel's last words, because Optimus is aware enough to not focus on how Sentinel's betrayal affected him personally, but how Sentinel had betrayed his own values.
This also shines a light on Optimus' behavior in "Age of Extinction. Because of everything that had happened to Optimus and his fellow Autobots in-between films, with Sentinel being proven right about Humans in part, Optimus was this close to betraying his ideals at several points in the movie, and that close to becoming just likeSentinel Primeor Megatron. The fact that Optimus ultimately manages to stay true to his ideals, and not betray them, shows just how different he is from both Sentinel and Megatron.
In the first movie, Optimus Prime transforms from vehicle to robot mode very slowly and deliberately when meeting Sam Witwicky compared to his later transformations. It's highly likely that he did so for Sam's benefit, to allow him to process what was going on without feeling threatened.
Compare Optimus’ first transformation with Barricade’s, Optimus does so slowly and deliberately, making sure nothing happens too quickly, and when it’s complete, he slowly leans in to ask Sam who he is. Barricade on the other hand does so abruptly and violently, and follows it up by roaring at Sam, chasing him, then loudly interrogating him about the location of his grandfather’s glasses.
Whilst the Decepticons are mostly immune to our weapons, they can be hurt be extremely high-temperature explosive rounds. Now, some people still wonder how our weapons could effect robots with metal skin and armor far tougher than any of our alloy. And how could they be? Well, the first film explains that most modern tech, including weaponry, was produced by studying Megatron's inner-workings.
The movies get a lot of flak for how effective the American military is against the Decepticons, sometimes even moreso then the Autobots who have fought them for ages. This actually would make sense, because the Decepticons have had all that time to get used to Autobot tactics and adjust their own (the stalemate this causes is actually the entire plot of the franchise!) and with Earth tactics being alien, new, and unexpected. America would naturally be the most effective, with them getting their tech from Megs and all.
The Ameicans are shown teaming up with other militaries and governments as well, if only because the Decepticons aren't selective in where they operate. In addition to the tactics issue, also recall that the humans have the Decepticons heavily out numbered, and are fighting on their own turf, Paranoia Fuel form transforming enemies aside. The Decepticons by nature of where they are fighting are always in danger of being on the receiving end of a Zerg Rush, where the Zerg have fighter bombers, tanks, and rocket launchers. And that is before the humans tailor their infantry tractics ot dealing with Cybertronian enemies in the third film.
Humans are also a fraction of the size of most Decepticons. Smaller targets are harder to hit. Sure, if a Decepticon does hit a human that human won't be there anymore, but once that human is armed with weapons that can actual deal damage to the Decepticon, the fight becomes, hm, deceptively fair.
The gripes about how all machinery that is affected by the Allspark transforms and acts somewhat hostile can be negated by the fact that every piece of technology on Earth was reverse engineered via Megatron's makeup, so technically everything is more or less another version of Megatron once brought to life.
Alternatively, one can argue that the Allspark simply gives life to the machine. It's possible that something else (Vector Sigma?) is needed to give the newly activated Cybertronian higher intelligence, such as a sense of morality. As a result, the Allspark-imbued machine is simply a primal beast, working on preserving itself in an alien environment.
Adding to this, the newly formed Cybertronians might have the capacity for higher intelligence, but lack the knowledge and life experience to really have intelligence. Go back tens of thousands of years, pluck a caveman out of it, and drop him in the middle of New York and see how many seconds it takes him before he goes ballistic.
When the Wreckers were first revealed, a lot of fans thought they would have redneck voices. They don't; instead, they're British (Cockney to be percise). So why do they have alt-modes as NASCAR Sprint Cup Series cars? Because Daytona International Speedway is about half an hour away from Canaveral, and engineers like them would want top-notch vehicle modes.
Optimus is criticized for not overly reacting when Jazz dies, or for that matter many of the Autobots who die. But remember that the Autobots and Decepticons have been at war for a long, long time. Optimus has likely been through the deaths of a lot of friends. It's not that he doesn't care, he's just been through too much to openly mourn anymore.
Even among his fellow Decepticons in the first movie, Blackout's design stands out as being one of the most overwhelming and hostile. While he has the monochromatic color scheme and jagged shape language expected from those of his faction, the sheer amount of helicopter parts surrounding his physique makes it difficult to read as humanoid. It makes him even more visually unfriendly than he would've been otherwise, and there might be a narrative reason behind this. Not only is Blackout the first Transformer shown in the series, but his introductory scene is a terrifying one, showing him slaughtering humans left and right without breaking a sweat. His design could be an embodiment of a human's first assessment of a hostile Transformer: intimidatingly huge, nearly incomprehensible, and packing more than enough weaponry to satisfy his blood-lust.
Considering the state of Cybertron and the Transformers race in the live-action series, being a Child Soldier is one of the least of their problems.
Keep in mind that pretty much all Cybertronians are millions of years old. Skids and Mudflap are likely just immature.
And the novelization of the third film has them killed by Sentinel Prime.
Soundwave's been on Earth since the nineteen-seventies, at least, keeping an eye on humans, and killing with impunity. And we never hear what happened to John Keller, Leo Spitz, Maggie and Glenn, did we?
Compared to much of the other Transformers "universes", there is little talk in the films about energon, aside from a few references to "Energon Detectors" and as a replacement word for "blood". (i.e. no allusions to secret stashing of energon off Cybertron, the existence of natural sources of energon on Earth or even the manufacturing of synthetic Energon that dominated G1.) Starscream even mentions in Revenge of the Fallen that the Energon shortage is making reproduction impossible. Given that Cybertron is a dead world, and the only known remaining source of Energon is the Matrix of Leadership '' does this mean that aside from the 9 Autobots on we see in the film, the entire Autobot race is slowly starving to death?
Speaking of reproduction, we do see some surviving hatchlings in Dark of the Moon, hatchlings whose sole source of care and food are Decepticons such as Megatron. No-one else even knows where they are. So, since almost all of these Decepticons are now offline, what happened to the hatchlings?
The one everyone's brought up: Sam being subject to Chuck Cunningham Syndrome starting with Age of Extinction coincides with the government deciding that all Cybertronians must die, Autobot or Decepticon, and not worrying too much about who's in the crossfire. Uh-oh! The Last Knight takes it out of the fridge - Sam's picture is seen as one of many of the "Order of the Witwiccans." A new character is the last surviving Witwiccan. Sayonara, Sam.
Sci-Fi Writers Have No Sense of Scale: Sam's body reads on a Geiger counter at 14 rads when Simmons scans him. In the few days that he's spent with Bumblebee, Sam has incurred a minimum of 47 times his annual dose of naturally occurring radiation, enough to measurably increase his lifetime cancer risk [...]
An easy line to miss during Blackout's intro—Colonel Sharp receives the incoming tail number of the unknown chopper as 4500X. He orders it cross-checked several times just to be sure, as 4500X was reported shot down months earlier. The tech somberly notes he already did: a friend of his was on that chopper. Knowing that 4500X was previously lost, presumably with all hands, Blackout showing up in its guise is akin to someone digging up a grave and wearing the burial clothes to sneak into a city—an ill informed plan at best and a deliberate desecration at worst.
it was the closest they could get to a literal translation.