But really, once someone attempts to kill you, you should have a free pass to kill them as much as you want. Overstepping the chain of command or whatever is an IC issue, not a rules issue. But I was told that this is not allowed, and was formally warned for it.
Next time don't bother capturing him, just finish him off as soon as he attacks you so it's self defense.
This. If you capture a bad antag, you've basically fucked yourself. If you kill them, they'll whine in ahelp about it. If you keep them confined for the rest of the round, they'll whine in ahelp. If you turn them over to incompsecurity, they'll make another poorly executed attempt on your life as soon as they're released. Just kill the fucker the first time and be done with it. Incidentally, this makes me wish that theft and assassination were tiered rather than equal, so only someone who's had a few antag rounds trying to steal stuff can be given an assassination target. A botched theft seems less likely to lead to whining than a botched murder.
Antag lessthanprotip: If you're playing antag role with the mindset of "What is the optimal method for completing my objective?" rather than "What is the most entertaining (for both the crew and myself) method for completing my objective?", you're doing it wrong. It's okay the first few antag rounds, but once you know what you're doing,
make it interesting. Goodness knows I fail at that often enough, I need more practice with hostage-taking and bomb threats as opposed to just blowing crap up. I've heard it put by someone (I think Hans) as being something akin to being a GM for the station that round: your job isn't to complete the objective, it's to make the round fun; you aren't playing against the other players, you're playing together, with the shared goal of a enjoyable experience.