In your first case: it is later revealed that the reason for A's action was B making threats of violence towards their kid and about to realize them.
The problem with third parties passing judgement is that they often lack full context.
The problem with third parties passing judgement is that they often lack full context.
Any system is going to be imperfect. The question is, is it better than things currently are?
I fully agree with you that when judging a specific occurrence, the context is really important. Judging completely without context, on the other hand, is good way to know whether the action is something morally acceptable.
In this case, the 'goodness' of the action clearly depends on whether you like the target. So I'd argue that this really is not a good thing in general. It's slightly worse if it hits the 'good guys' and slightly better if it hits the 'bad guys', but it's not good either way.