Objectively no its not justified.
Subjectively yes I would do it and feel (convince myself) justified, both as defence and as revenge - though of course that's just conjecture as I've never had sufficient provocation (and X-fingers never will).
I do know, however, that I will react very violently if my bluff is called. For example, a total stranger kept putting ice down the back of my dress in a pub, and after I asked him several times to stop he asked what was I going to do about it. So I burnt him with my cigarette - just to avoid backing down. Not very clever, as this resulted in a fight etc, but I know that I'd do it again in the same situation. If the stakes were high enough I'm fairly sure I'd kill.
I've also seen that adrenalin thing that some men get when spoiling for a fight. Both my husband and his brother can get very hyped up when riled (not often) - and their breathing changes and you can see by their weird eye-focus thing that there will be trouble. Its a very powerful state of mind and there is no rational thought - so who knows what seems justified under those conditions?
Civilisation is sometimes a very thin veneer isn't it?