The infamous opening scene where Herbert West tries to use a prototype of his serum to save his mentor, Dr. Gruber, who is dying from a heart attack. Instead, Gruber leaps to his feet, screams in hideous pain and clutches his head before his eyeballs burst like blood-filled blisters and he collapses dead on the floor. And West doesn't even bat an eye at the grotesquerie, instead cursing his lack of success.
While most of the violence is quite over-the-top, the appearance of the morgue zombies were painstakingly based on actual corpses of accident victims, and as such are disturbingly realistic and extremely scary.
The movie heavily implies that being brought back to life leaves the test subjects in unbearable pain (physically and mentally—the longer the body has been dead, the worse the resultant pain), which may very well be part of the reason that they are more like feral beasts than humans. They die and, after West's chemical restarts the body, literally wake up in a living hell where pain is all they know.
Dr. Hill. Just Dr. Hill. His ability to manipulate people in life is already unnerving (implied in the original screenplay to be actual hypnotic powers - which explains why West temporarily cowers and gives into him after Hill tells him he'll do as he's told) and once he's dead and re-animated ... oh boy. The creepy post-death voice doesn't help.
Even when he was alive, he still a creepy vibe about him, which only intensifies when Dan and Megan find a folder of Megan in his office containing hair clippings, pictures, and other "keepsakes."
The infamous "giving head" scene. Megan is knocked out, stripped, handcuffed to a table, and molested by the re-animated Dr. Hill. Once she wakes up, she can only scream and try to push away Dr. Hill's severed head as he tries to perform cunnilingus.
