If I had to, I would.
Now to define having to:
1. My life is in mortal danger.
2. The life of a loved one is in mortal danger.
3. The life of an innocent is in mortal danger (ie., children, elderly, non-combatants)
Underlining all of that is the assumption that I'm unable to stop the person without killing him/her. I've had knives pulled on me in the past and on only one occasion was I slashed. Every time, I was able to disarm the threat.
Guns are the wild card. They're primary function is to kill. I've personally witnessed acts of violence with guns. I've seen the damage that can be done by two men with the means and intent to do harm.
That said, if I could have killed them and prevented their shootout with the police, I would have. The only people who were killed that day were the two gunmen, but so many innocent people were shot.
I doubt many of you may remember, in 1997, two gunmen attempted to rob a bank in Hollywood, California. My father drove my family right past that bank when the first LAPD officers arrived. He then made a u-turn, hoping to see something exciting. It was during the second pass that the first shots were fired. Real freakin' exciting.
Anyway, I would kill only to preserve the lives of the innocent.