What planet are you from? First of all, Armand should have disabled him and called you, or me, or someone, or just walked away. Costigan wasn't trying to kill him, and Armand is stronger then him. Second? The only reason it wasn't murder was because Donny showed up and stopped him from choking him. You don't get to try to say your inmate trying to kill someone was totally okay because someone happened to hear the fight and stopped it.

And I know why Costigan did it, that's why he's not in Zero. Maybe you should do your job and ask Armand.