That’s a difficult question to answer since the research into “fantasy” of any kind is minimal overall, and doesn’t specifically delve into the “dark” fantasies outside of people that are known already to have them. So you end up having trouble finding reliable information, and even that’s for a given value of reliable since replication efforts in psychology and psychiatry aren’t exactly booming.
Then, the specific question you asked has “violent sadistic” as a combined criteria. It’s definitely possible to have violent fantasies without sadism, and sadistic fantasies that aren’t violent (unless you’re one of those assholes that insists on stretching what the word violent means far beyond sense).
With all of that said, I can only give anecdotal information regarding people I know well enough to have the kind of honest talks that they would admit this kind of thing. In the way you asked, based on that, I would say that the incidence of violent sadistic fantasies is low. I would say the same about sadistic only fantasies. But violent fantasies?
If you include things like wanting to punch someone just one good time, I would call that fairly common, with the frequency decreasing as the level of violence goes up, and/or the duration of the violence goes up. But does that actually reach the kind of ideation that wolf be called a fantasy.
Myself, I say not usually. A fantasy is a conscious thing in most usages. It’s something you think about, imagine, and focus on. The kind of violence most people have in their head is impulse driven, or stress driven. That isn’t really conducive to fantasy. It’s like wishing you had a nice cup of coffee isn’t the same as imagining brewing up a pot of excellent Jamaican blue mountain when you get home and just sitting on the porch drinking it while the sun goes down. One is a passing though/impulse, the other is a fantasy.
Same analogy (or is it metaphor? Damned if my brain is pulling up the difference right now), but using sex. A passing thought that someone would look good naked isn’t the same as picturing them naked and imagining what you’d do together. The first is not a fantasy, the second is.
Thus it is with violence. A sudden urge to pop your boss in the nose isn’t a fantasy. Imagining beating the shit out of them is a fantasy. Until you’re constructing the scene with intent, I say it isn’t fantasy at all.
Now, me? I’m prone to violent impulses. My PTSD stems from violence (civilian) in childhood and as an adult. You get attacked and injured enough, you react differently to perceived threats. Someone cuts too close to me when driving, there’s a flash of wanting to kick them in the nuts/twat. But it’s a flash. For me to seriously want to fuck someone up, it has to reach a level of threat or problem much higher. Even that isn’t sadistic in the truest sense, since it isn’t about me enjoying the process, but that’s splitting hairs. There are people in the world that I would enjoy beating the fuck out of, but it’s my experience that I’m kinda rare in that. Most people would avoid it even if they knew they would get away with it.