For the first film, the death of Michael Ironside's character, Lt. Rasczak. His agonized "you know what to do" just before Rico is forced to shoot him doesn't make it any easier.
Dizzy's death; even if you hated her and are a Rico/Carmen shipper, it's still horrible to watch.
Made even worse by the fact it was probably Rico's fault. Any infantryman is taught that you NEVER pull out shrapnel as you will make things much worse. You bandage around it and leave it for a surgeon. Diz might have lived if Rico hadn't mutilated her insides.
Her funeral and Rico's accompanying eulogy is also this, but not because of anything like Rico's emotional sendoff to his Second Love, it's the lack of an emotional response from Rico. He doesn't give his eulogy as though it was a friend he's known since he was a teen and someone he had gotten into a romance with, he gives a completely detached speech about how Dizzy was a good citizen who gave her life for the cause, effectively erasing all that she was in the name of exploiting her death to further the war against the bugs. Rico has basically lost everything by this point and he's become such an Empty Shell that he's become nothing more than a mouthpiece for the propaganda machine.
Rico's father is extremely disapproving of his son joining the army, to the point of claiming that he's disowning him and cutting ties with him when he refuses to change his mind and go after Carmen. However, after the aforementioned accident and the flagging punishment (something that Rico's father bitterly foreshadowed by claiming "I'd rather take 10 lashes in public square than see you ruin your life!"), Rico decides to resign and calls his family, and despite his dad asking him where's his uniform, he accepts Rico's apology and reassures him that he can come home, and even before Rico admitted his mistake his dad said "It's just good to hear from you. How are you, son?" Too bad that soon after the Bug Meteor prevents any family reunion, and throws Rico back into a revenge-driven Bug War that will end up turning him into another cog of the system.
The one silver lining of Rico's failure during boot camp is that at the very least he managed to have a final exchange with his parents and being in good terms with them again. Imagine if Johnny's father angrily shouting that he's cutting him out remained their final interaction.
Zander's death. Yes, he was chasing after a woman in a relationship at the time, but he was an otherwise upstanding officer and man. In fact he risked his own safety to give the Roughnecks cover fire to retreat from Planet P when he could have sat safely in the cockpit and he kept his word when he fought Johnny about disregarding rank when he could easily have had Johnny kicked out of MI or lashed again for striking an officer. To see the guy get his brain sucked out is both tragic and horrifying.
The Arachnids are undeniably feisty Insectoid Aliens, but it's a bit hard not to feel sorry when Carl triumphantly announces that the Brain Bug is literally afraid, and then during the final montage scene we see how it's being experimented on with nasty drill-like injections that are clearly paining it a lot. And remember, It Can Think, and the barbaric treatment of the human scientist is probably causing it unwarranted trauma.
Look at the movements of the Brain Bug when Carl reads its mind, the big guy is clearly cowering in fear and trying to shrink down while being touched by Carl and its shiny multiple eyes focus on him.
During the clean-up operation of Tango Urilla, the Roughnecks fire down at a surviving Warrior Bug which wearily looks at the humans with one shiny eye, almost attempting the closest thing to Puppy-Dog Eyes while the scary noises it makes now sound much more pitiful and high-pitched, but Sugar just proceeds to furiously riddle it with bullets and has to be stopped by Dizzy.
It mostly applies to the first movie, but seeing what humanityhasbecome is hard to watch.
