Human skeletons have long been a staple of Halloween and haunted houses.
It is only just now occurring to me that there is a good evolutionary reason why skeletons -- or just skulls -- are such a scary sight: they often signify that there is something in the vicinity which can kill you.
Maybe it's a nearby tiger which has developed a taste for humans. Maybe it's a nest of vipers which live close by. Maybe it's a poisonous water supply. Maybe it's a serial killer. Whatever it is, it represents a potential danger to you.
The skeletons themselves are obviously not dangerous to you, though some people seem to find them creepy in a ghost-like, zombie-like way. It's the fact that they signal a potential danger lurking nearby which proved fatal to the person that skeleton used to be, and which could prove fatal for you. (This is why we don't feel the same jolt when we see a longhorn skeleton, for instance.)
Evolution works in a logical way, but the instincts it breeds may not necessarily seem so. For instance, the sexual instinct has been bred into us as a means of procreation. But when you see an attractive member of the opposite sex, you do not think, ah, I really want to procreate. Your thoughts are more along the lines of, I want some sexy fun with that hottie. It doesn't matter that you're not thinking of screaming little tots; your instinctive desires will result in them, and that's all that matters.
Likewise, when we see a skeleton, we don't have to run through the possibilities of what may have caused that human to die. All we need to feel is a desire to flee, and if we think that it's the creepy skeleton itself which is dangerous to us, no matter: the important thing is that we flee.
So, over the millennia, we developed an instinct to feel scared when we see a human skeleton.
There are a lot of different types of people who have taken advantage of this instinct. It's why the pirates used the Jolly Roger, with its skull and crossbones, as their flag: to scare merchant ships into quick submission. It's why guys who want to appear badass will get tattoos with a skull and crossbones motif. It's why hazard signs will sometimes also feature them: to instill an element of fear so that you take their warnings more seriously. It's why skeletons are featured in horror movies.
Evolution has selected human beings to react appropriately when a sign of danger appears.