If you’ve never read Barth’s discussion of ‘killing in self defense’, you ought to. Especially now, when killing seems to be something of a sport for some and self defense is urged to such an extent that Christian people are told, with straight faces, that it is proper for them to meet violence, or even potential violence, with murder.
Barth’s treatment of the subject of self defense is the best in all of theological literature, and can be found in his Church Dogmatics, III,4 (p.427ff).
He writes, for example, quite provocatively
This cannot mean that somewhere and somehow… the imperative reaction of self-defence and therefore the primitive instinct to protect our possessions is right after all. As forcefully as it can, the command of God tells us that this instinct is wrong and not right (p. 433).
Again, the entire section is absolutely brilliant. Do read it before you say yes to retaliation and vengeance.