When I was much, much younger -- late teens and early 20's -- I found that I was capable of generating some really gruesome fantasies. Scared the crap out of me. I wondered what kind of awful person I must be, to come up with that stuff.
Eventually, I came to see personal fantasy as a "free fire zone" in which hurts, irrational anger, destructive impulses, extreme egocentricity, etc. could be discharged without doing any real damage. The emotions provoked were real enough to me, but nobody else was affected. And I realized that I had absolutely no desire whatsoever to act out in real life -- even if I didn't care about the consequences for others (which I did), in most cases even making the attempt would have ruined it for me. They worked, quite literally, only as waking dreams. I came to see it as a "no hurt/no foul" situation and quit worrying about it.
Whether because of that realization or because my life generally improved quite a bit, my fantasy life became a brighter place. The occasional dark image rolls through once and a while, and I usually indulge it for a bit, but eventually grow tired of it. Sometimes I can see that it relates to a particular situation or ongoing frustration, sometimes not. Eventually, it goes at it came -- both unbidden and undismissed.
Far from being the "thought becomes the deed" person I've heard moralists describe, I'm apparently quite capable of spinning stories in my head as a substitute for action -- indefinitely. Never tried writing them down -- not really afraid, I just don't want to. And I suspect most people would find them tedious as much as anything.
Obviously, this is a very YMMV situation, but that's my experience, which is what I think you were asking for. I hope it helps.
