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.
"I love you, Sarge!"
The way the ghost is right before he blows up the Amerigo.
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.
It gets 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" mission, the colonial leader succumbs to an infestation and tries to kill Raynor, in a lovingly-rendered cutscene. Thank you, Blizzard, for that charming image...
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 put into absolute Uncanny Valley