I used to not know the answer to this question, but after being physically attacked by a drunken asshole last year, I do now. I'm not a violent person by nature, or even an aggressive one. But I have a strong sense of justice, and when it is violated I can instantly fly into a rage.
When I was jumped, I entered a different frame of mind and could have easily stomped him to death without realizing the severity of my actions. In the flashpoint of that moment all that entered into the decision was that I was minding my own business, and this person had created the situation. I felt like my attacker had given up his right to mercy when he showed me none and made me defend myself. I didn't feel responsible, I felt intoxicated with righteousness.
Given time and consideration, I don't know if I could kill. Taking someone's life is not a decision I feel qualified to make. But I know that in a quickly unfolding and stressful situation I absolutely could, and I would wager that nearly all of us could. Even though we enjoy the polished veneer of a society of laws, we still have long-repressed animal instincts that served our ancestors well. And I think they can be accessed easier than we think.