I'm assuming this is, in part, a response to to Buffy, Season Six, Villains, aired last night on Channel Seven. If not from him, then certainly from me. A powerful episode... and Willow is pretty serious about that vengeance kick. Understandably so... but the morality of it is a little less black and white than Buffy seems to see it. Buffy seems to say "it's a human, therefore it is not okay to kill them because they are the 'natural order of things'." Admittedly, she did go on to say, "there are human means for dealing with this..."
My thoughts on the subject are as follows. There's no magic line that says "thou shalt not kill". Humans are not special. I am regularly responsible for the death of animals (and would be happy to kill and prepare them personally, if it was necessary) so I may eat, and ditto for the death of plenty of plants. Every day, millions of bacteria live and die in my intestines. Life feeds on life. The only reason not to kill humans is cultural and pragmatic - other humans are likely to seek retribution.
I sat down a while ago, and hashed out whether I could kill a human or not... and I've come to the conclusion that I could, if they were attempting to engage in applying forces that carried risk of killing me or people I care about.
The reason I had to think about that is because, well, I am sufficiently well trained to be able to kill the majority of humans, with my bare hands, without significant difficulty. So, I've trained my response-patterns to be "do-unto as you are about to be done-unto" - if somebody is trying to hit me (and I'm aware of it in time), they'll get hit about as hard as they were trying to hit me, and I won't get hit. If they're using potentially lethal force (a knife, or simply being big enough to hit hard enough to do lethal damage), my response patterns will scale straight into "kill or die" with zero qualms.
Non-immediate responses, that's different. I'm unlikely to seek direct vengeance, if I'm not around at the time an attack is occurring... but that's simply for pragmatic reasons. There are normal human-societal means (police, courts, etc - no they aren't perfect, but they are the accepted societal mechanisms, just as Danegeld payment was for the Viking culture) that are available and should be tried first.
If they don't work, and I'm convinced it's necessary to put someone out of my misery, then, I might well seek personal vengeance, if I feel the likely consequences are worth it. That's not a likely occurrence, though.