I have killed in dreams before, and by two methods in particular.
I would have dreams where there was a sword hanging from my belt with a smooth curve to it, and whenever I unsheathed it I could feel an immence force welling up through my body. In dreams I am unstoppable with a blade, and I have killed both defensively and offensively. These dreams are rare and far between, so I dont will it to happen.
Another single dream where I had commited homicide had to have been the largest 'mass murder' performed by myself dreamwise. I was sitting in a college classroom oddly enough, it seemed off and stark. For some reason the teacher has a problem with me. She made herself out to be incredibly stupid and insulting, and I simply stood up in place and lashed back. I had a weapon, a chain with a weighted pike on the end. I can still remember whipping it around, slashing and ramming it through the classroom's occupants... Luckily there had been no one recognizable there.
Well, whenever killing does come up in dreams, I simply feel empowered, the same way I feel when I fly in a lucid dream. At least I've never killed on purpose in lucid dreams (or at all for that matter).
[Edit] And I just want it to be known that I am not proud of myself for killing in dreams, but neither am I regretful, since the people were not real and that I would never duplicate those actions in the real world.