Hmm, the human mind is very complex. So there are all kinds of things. I'm not having these thoughts involving actual people. But I can let my creativity run free and imagine all kinds of fantasies. I also read books and picture things. Or watch movies that contain violence. But I separate that from reality. With that said... There are things like intrusive thoughts. A lot of people for example report being haunted by the intrusive thought, they could (while driving) at any time yank the steering wheel, crash into that concrete bridge pier and die. That might be uncomfortable. I think it's called "L'appel du vide" or "the call of the void". But I think it's connected to the brains ability to weigh options, picture outcomes and assess risks. We can also assess dark scenarios. That in itself isn't bad. But it becomes an issue once you lose control or it starts affecting things. Same applies to other disturbing thoughts/fantasies. I'd be super careful if it involves real people. But I mean we do that. We sometimes also fantasize about positive things. Like having sex with someone who is out of reach. Maybe that's practically the same thing. But it comes with issues.

So yeah, I'd say things like that happen to mentally sound people. It depends on the extent and how you deal with it. And there isn't anything like a "mentally sound" person in my opinion. Most people I've met are crazy (in a good way) one way or another. And life isn't strictly one thing or another, either.

One thing that makes me worried though, is you mentioning the word "sadistic". I can't relate to that. I'm not sure if it's a common (or healthy) feeling. I think I've never experienced any urge to be sadistic towards any living being.