No, I would not kill a Nazi. I would never contribute to any violent efforts of any sort. The only time I would ever even consider resorting to violence would be if I was being directly threatened with violence at someone else's hands. And then I would only try to maim, and get free, not kill the other person.
There is nothing humanistic about taking a moral stance against violence. I believe the right to be free from violent harm is a basic right that we should extend to people (like health care, among many others)--not because I believe humans are essentially good, but because I believe humans can be very very violent, and this impulse is not conducive to healthy social interaction. Ever. I believe the negative tendencies of humans need to be fought against, not surrendered to for the sake of some cosmic Ideal (i.e. communism).
Humanism is not identical to just any moral stance. Humanism is a doctrine of essentialism that makes claims about universal traits that make humans what they are. Communists are humanists, who believe that humans are essentially "equal" and all essentially good--that it is only capitalism or unjust power structures that cause all human problems.
I am not a humanist, because I do not believe there is an essence of humanity. I believe humans are determined by several factors--biological/genetic, social, economic, linguistic, cultural, etc. I believe humans display a whole range of behaviors that is never static, unchanging, or universal. If you change the factors that determine human behavior, to the best of your ability, by refusing to support, or abide by, or further, or participate in negative, destructive behaviors (on interpersonal, cultural, and global-social levels), you change the way humans are likely to behave. Humans are not essentially any one thing. Humans are all different and always morphing into different beings.
In order to make people what I want them to be--peaceful, non-violent, non-aggressive, not petty, not psychotic, not mass hysterical creatures--it is necessary that violence be avoided at all costs. The costs of violence on human social interactions is just too great in terms of irreparable damage to be worth it, not even with the best of intentions. In fact, I believe that the means are the ends--if you are forced to use violence to bring about a regime, that regime will be violent.
Two wrongs don't make a right.