Guards can kill a prisoner if they:
Kill a guard in an lr that is not a deathmatch (Guntoss is not a deathmatch)
Intentionally start attacking a guard who is not in the lr (Crossfire doesn't count stop getting in the way of last reactions and then blaming it on the prisoner once you die)
Use an lr weapon to attack guards after the lr has ended (last pris shooting at a guard with a sniper after it ended)