Honestly? I think the thought/fantasy/idea of killing some is far more common than most would like to admit (especially if they've done us wrong). I think it's the actual act of going
through with it all that's by far more rare!
I've had murderous thoughts for years, ever since middle school and maybe even beyond that. Most of it has to do with my fascination with violence...I've see beautiful guys that I'd fantasy about tearing apart---subjecting them to various cruelties. Whipping their backs bloody and raw with a massive bull whip and then dragging my tongue along the bloodied welts, taking a sharp razor or knife and carving into their hips and chest. Biting them black and blue and tearing at their lips, and for the grand finale tearing out and devouring their heart. A lot of time I'll fantasize doing these things to individuals who more or less deserve them...Serial killers, rapists, and child molesters...The added element of doing this to an 'evil' individual whose subjected other to their own will makes it that much more hot to me. Another scenario that always runs through my head is catching some junkie jacking off in a public restroom and beating him black and blue, then forcing him to 'service' me in a variety of ways
I've had quite a few self-mutilation fantasies as well that usually creep up on me when I least expect it (like at work) One that I've had for months is taking one of the large box cutters that we use at my job and burying it's blade deep into my throat, slashing it from top to bottom. I can almost feel the hot blood bumping out of my jugular when I imagine this, and the horrified faces of the unfortunate on-lookers as my hot blood splatters their slack-jawed stares. Another one involves smashing my skull in with a hammer.
It sends chills up my spine to imagine these things with the up-most of my ability, but I know deep down I would never be able to visit these horrors upon myself or others. Good thing too, otherwise we'd be like the rest of those maniacs out there (That, and I think the reality would never quite live up to the fantasy...)