Huh, surprised I missed this thread until now.
Anyway, I do have more than occasional sadistic fantasies. I've done a good deal of introspection, and figured out some of the patterns.
The first is that I have to be in a very specific, and specifically very good mood before I can get any real enjoyment out of sexual fantasies involving hurting or humiliating little girls. If my mood isn't quite right for it, a sadistic fantasy will tend basically get me nowhere and end up leaving me drained and in a worse mood. I tend to think of it in terms of needing my ego to be in a really good place before the fantasized rejection that is a rape fantasy stings less than the enjoyment I might get out of the domination. Whereas I'm always up for a happy, consensual fantasy. Fantasies involving consensual S&M are usually somewhere in between, since they don't sting my ego, but do require more effort imagining the setup.
The second pattern relates to the content. Regardless of my mood, I never like abusive fantasies that involve breaking the skin, even when things get to the point of murder and necrophilia. I enjoy humiliation and psychological abuse fantasies much more than I do physical abuse, and rape by deception fantasies more than forcible rape ones. When I am in the mood for physical brutality in my fantasies, I tend to limit it to beatings, bruises, and the occasional knocked out tooth or broken arm, with strangulation ending up in there somewhere, but never stabbings, burnings, or flayings.