Everyone agrees that the Zerg were pretty messed-up. But the Infested Kerrigan may have been pushing it, especially in her introductory cutscene. Similarly, a few cutscenes of Terran characters being killed by the Zerg were like something out of a B-Movie, complete with Southern accents.
Even the introductory prologue cinematic is no slouch. A bunch of Terran salvage scavengers happen upon a bunch of debris in orbit of what is later revealed to be Chau Sara. They launch to pick it up, and communicating over a grainy video and radio connection, one of the techs notes that "We got Terran for sure, I don't know about the rest", which is a major Fridge Horror implication after a second once you realize, what else could it be? Then the video feed glitches out, while a massive Protoss warship — so huge that you can't even see the whole thing, let alone make out what it is, whether a Carrier or something else — arrives Behind the Black above the scavengers' ship. The guy crewing their ship manages to get the video feed back, only to be greeted with the alien sight and a rapid Scare Chord ensues, as the scavengers erupt over the radio in panic and the Protoss ship pours a beam of plasma down upon the planet through their vessel. The player is treated to the sight of the lead scavenger in his cockpit, his head exploding in a bloody mess on-screen as his ship is ripped apart and the men out in space likely face a similar fate within moments.
The first major cinematic in the original game has a pair of militia patrolmen ambushed by a pack of Zerg in the middle of the night. Made even worse when the immediate next mission has the player's base under regular siege by a horde of Zerg, against which they must defend for thirty minutes, in a major Difficulty Spike for a new RTS player.
Sarge: Its a Zergling, Lester. Smaller type of Zerg. They ain't be out this far, unless ... oh, shit. [both soldiers turn around to see a pack of Zerglings accompanied by a Hydralisk, which have already surrounded their vehicle] Lester:I love you, Sarge! Sarge: Huh? [the Hydralisk roars and charges them] [crunch] [cut to the two men's fallen flashlight on the ground as it flickers out]
Even Arcturus Mengsk's inaugural speech in the closing cinematic to the first game's Terran campaign features such sights as a battlecruiser helplessly plummeting on fire into a mountainscape as flights of interceptors swarm towards it, Zerg Guardians hovering over burning city ruins while Mengsk talks about "friends and loved ones consumed", and a severed arm still holding a smoking cigarette, trailing globules of blood as it does some Dramatic Space Drifting across the screen among debris while more Protoss interceptors fly by in the background. It certainly pounds in how likely screwed humanity is, caught in the crossfire between two other powerful races that have either substantial technological or numerical advantages over Terrans in the Koprulu Sector.
The Battle on the Amerigo. It starts off with Nothing Is Scarier as the Dominion squad infiltrates the derelict science vessel to demolish it with a nuclear bomb, in a scene that wouldn't be out of place in Alien. The squad reaches their destination and one of the soldiers comments in a Tempting Fate attitude and cracks open a can of beer — right before his head is abruptly impaled from behind and dismantled by a Zerg claw and he gets hoisted out of sight by something while still audibly gurgling in his death throes. Seconds later, a pack of Hydralisks, presumably left behind by Kerrigan in the preceding mission, then swarms into the room and starts administering a Curb-Stomp Battle to the rest of the small squad (see the page image above, where a Marine runs out of ammo and starts firing his sidearm point-blank at one in desperation). One Marine manages to bellow to the squad's technician, who has himself been frozen in place by pure fear the whole fight:
The entire Protoss campaign is one large Darkest Hour in the entire storyline, with the Terrans having lost multiple worlds to the Zerg and the Zerg now rampaging across the Protoss homeworld of Aiur. The Protoss have a chance to help end the war earlier but Civil Warcraft breaks out with the return of the Dark Templar and the Conclave won't set aside their differences and actively impede the war effort to defeat the Zerg. At this moment of the story, the possibility of losing the war to the Zerg seems very real.
The Paranoia Fuel of the cinematic showing a Protoss attack on a Dominion military encampment on Char. First, a lookout spots a broken Dragoon making its way through a pass near the camp; after waking up his sergeant, the unit's mobile artillery piece obliterates the Dragoon in one shot. The lookout starts whooping and cheering in celebration — when a fireball abruptlydestroys the lookout and his watch post, and another Dragoon appears (seemingly decloaking) a few feet away in front of the sergeant, who in turn barely has time to Death Glare as the automaton in front of him burns him to ash with a ball of plasma. (Bear in mind, the sergeant is in fatigues and not even wearing Marine armour.) And behind this, a dozen or more Dragoons also appear, likely due to an Arbiter that flies overhead, and they casually turn the camp's tents into a burning wreck. Most of the platoon or company that formed the encampment probably didn't even have time to know what was going on, or were still asleep, right up to the moment they were killed.
The infested Terrans. There's almost nothing left of their humanity, and the do enough damage to kill any unit with their suicide attack. Specifically the point in the campaign where a marine asks, "Do you hear that?" and then one pops up right in front of you.
The concept art of them. They're so full of alien parasites they're bursting out of their space suits. The sprite is very low-res, so it's not as bad-except for their Unit Profile on the little TV screen at the bottom, where they're shown to be so mutated they can't be recognized as having human faces anymore.
The opening cinematic in Brood War. It takes place in a WWI-style trench at night. It starts with a desperate Marine getting ambushed and chopped by a Hydralisk before a Firebat fries them both, with the Marine still alive through the process (briefly). The Firebat promptly gets blown up by a nearby explosion, possibly from their own Vespene gas tanks. The cinematic then cuts to another desperate Marine barely rescued from two oncoming zerglings by another Marine. The first Marine asks the other one where the air support is. The older man, who has all but given up, wordlessly points at the Battlecruiser hovering above them, which abandons them soon after at DuGalle's orders, eerily similar to how Mengsk left Kerrigan behind at New Gettysburg. The view zooms out and out from the Marine staring skyward and his placidly sitting companion, as hordes of zerglings are rapidly closing in on the trench and the cinematic cuts to black.
The Zerg get even worse in the sequel. Better graphics mean more opportunities to give the Zerg stuff like sacs and pustules and tentacles and so forth. There's even a series of missions involving helping a group of colonists from a Zerg assault and infestation. The second mission alternates, every five minutes or so, between day and night. During the day, you run around blowing up creepy-looking but inert infested structures. At night, the former colonists come out and try their level best to eat you and your troops. Worse, if you choose the "bad ending" for the third mission, the colonial lead scientist succumbs to an infestation while trying to develop a cure and tries to kill Raynor, in a lovingly-rendered cutscene. Thank you, Blizzard, for that charming image ...
The Reveal that it's Amon who's behind protoss and zerg' uplifting is even more nightmarish. Khala, the unity so dearly cherished by protoss? A tool to enslave them... which is exactly what happens in Legacy of Void, as Amon made brainwashed protoss kill their brethren and all other life. The Overmind, the collective Hivemind center of the Zerg? It's deprived of its freedom of will, and made to serve Amon's genocidal plans. It even knew that after the Zerg would have exterminated the Terran and Protoss the Hybrids would destroy it, and hated this. Fortunately, it managed to work around his brainwashing.
The Hydralisks, in-story. Sure, the main heroes tear through them like nothing, but imagine being a regular marine - or worse, a non-armored person, dealing with these guys. Armed with blades / claws (depending on which game) that are driven by huge shoulder muscles and can punch through heavy armor, and also possessing armor-piercing projectile spikes and a wicked set of jaws. They're ambush predators, so you never know when one is gonna pop up from literally underneath you and rip you apart. And according to official data, Hydralisks are not only smart (by Zerg standards), they're sadistic. StarCraft II cinematics give three more reasons to fear them. First, those projectile spikes can punch through 2 centimeters of reinforced steel and are tipped with a powerful venom; one guy got spiked in the arm, and had to have the arm amputated to survive. Second, they hunt in packs, meaning that where there's one, there's generally at least two more. Lastly, they're huge; at full "height", they stand about 3 meters (around 10 feet) tall. Imagine one of these guys towering over you, ready to ram its claws through your chest and / or skull after it's done toying with you.
To add some Body Horror or fuel to this, the backstory says that Hydralisks were originally peaceful herbivores before the Zerg Swarm absorbed their DNA and turned them into what they are now.
Mira Han. While she has some funny moments, her eyes make her look creepy.
The short story The Education of PFC Shane shows us what it's like to be infested- and, through some zerg-induced flashbacks, to be resocialized. Both involve being forced to relive the most traumatizing, horrific events of your life until you beg to have your mind rewritten. It's even worse for Shane, because some part of him is still resocialized into being a model citizen, and that part is utterly horrified at what he used to be.
The opening cinematic for Heart of the Swarm shows just how horrific the prospect of the fighting the Zerg actually is. The pounding music and the waves of Zerg of all species shrugging off every Terran countermeasure and swarming over the city hammers in how immense and unrelenting the Zerg are in battle.
The Defiler's Plague spell isn't just Hollywood Acid (which would be bad enough), but a weaponized, cancer-inducing virus as well.
The Torrasque. As if regular Ultralisks weren't enough, this one won't stop even if it's killed.
Who says this? Samir Duran. He is making Zerg-Protoss hybrids. And he serves a bigger, still unseen master...