Seriously, I don't know what it is, but just by typing it I got goosebumps. It's always been one of those little things that makes my skin crawl. Don't ask me why.
Disturbed humans or humanoid-like things creep me out. Those with sociopathic tendencies, without conscience, those who get pleasure from torture, etc.
However, anything that does the same thing but doesn't look humanoid doesn't really creep me out. For example, the alien tripod thingies didn't freak me out in War of the World, infact I found them quite entertaining. But, had it been humans going around blowing that ominous horn and converting other people into fertilizing pulp, I probably wouldn't have been able to sit through the entire thing. (Now, of course, one might argue that, given my fear of disturbed humans, I shouldn't have been able to sit through a film featuring Tom Cruise..)
Women that murder pregnant women to cut out their fetus. To me, that is sicker than any jealous boyfriend or homocidal husband... Think of it. Cutting out a fetus..for what? Some dark psychosocial call that screams for them to have a baby, at all costs. Theres no escape from that. That shit is so dark and evil even I can barely admit that its real. It doesnt help that every time I read a story like that, I say to myself: "I can say exactly which person I know is most likely to do that some day."
Apart from that? Growing old. Not dying, i dont mind the thought of death. But I hate the idea of enfeeblement. To be barely capable of things that most of the population can do without thinking. Incapacitation. Mental degredation. To be trapped in a body that may not even move correctly.
My only fear is blood. It's mostly due to me being desensitized to most other things. I've seen people being beaten and seen people being killed. I've had a gun to my head before. I fear neither physical pain nor death yet I'm afraid of blood.