Well... stuff happens in the real world. You can still defend yourself. Sometimes you just have to make a choice between pacifism and murder. It's not like I would hesitate to murder someone that was threatening my family or friends, but I'd still feel really bad after the fact and if they had survivors I'd probably be trying to comfort them.
Ethics are arbitrary and invented. All norms are. Realizing that, I'm not comfortable telling another person that they should be punished for
anything, but sometimes we have to simply because there isn't any other choice because criminals cause so much harm to people. However, because we have recognized causing harm as something to avoid, we can't conscientiously cause undue harm to prisoners either.
You can still imprison people, but the process should be absolutely uniform, impartial, and as least sadistic and as least vengeful as possible.
Universal compassion isn't allowing an emotion into an argument that decries emotional judgments. It's simply the principle that glues the entire chain of reasoning together, and even though it's just as arbitrary as wanton vengeance, I believe that compassion leads to much more happiness and peace.