Honor is a concept shared between true men and should be respected. If your opponents act without it, then they are not men but animals. And you should put them down like you would any dangerous animal.

That doesn't give you full authority to torture and be a monster just because, just like it doesn't when you kill a bear or the like, but it does give you full rein to do what is needed to control or remove the danger. Its not evil to steal the eggs of snakes or poison entire colonies of insects, nor would it be when dealing with men who've lowered themselves to that level.