You can try to control every little thing to make sure everything is fair, or you can let your players play.
Seriously?
One player's character murdering another player's character in their sleep, when rules at the table either do not address PvP or expressly prohibit it - is not a "little thing."
It is something worthy of immediate discussion above game.
If players are into into it - great.
But if players are not into it - this is an out of game issue and needs to be resolved as such.