Their rebel status gone away is the "hard" thing for a guard to understand.
If the guard remembers that player rebeled (maybe he remembers the prisoner's name in chat, after he rebeled), well, he can kill him.
If the guard doesn't remember, well... He won't find any reason to kill a "normal" prisoner, who instead was a rebel.
All Rebelers of a round can be killed, even if their rebel status's gone away.
Surely, if the dead prisoner will start with "FrEeKiLLer!1!1" and so on, you just ignore him or explain him why you killed him, if you are sure that they rebeled (scrolling the chat can help you).