I didn't think this was unusual for the longest time
I'm a 17 year old girl with an above average IQ (130-135), but still somehow managed to miss the fact that some of my thought processes were odd (damn you, social awkwardness!)
I get bored, and I daydream. When I was little, I'd daydream that I was an animal (cheetah/giraffe/hippogriff/albatross/wolf/etc). For a while now, though, I've daydreamed about how I would kill some hypothetical person. I take into account habits of the made-up person (scuba diving, driving alone, drug use, suicidal tendencies, etc), victim specs (tall/short, heavy/light, shape, intelligence, and so on), motive (degrees of punishment, need for removal, etc), setting (Florida summer, out at sea, spring in Mexico, Australia, monsoon season, whatever), and all the specifics. It can be very detailed.
I also daydream about creative ways to booby-trap a house, and what mechanisms would be necessary for it to work.
I sometimes plot out both in detail, but it's just mentally. And usually the beginning of the train of thought is unintentional, sometimes unavoidable. I'd never ACTUALLY do any of what I think of.