Why not just have an "intent" flag that the player sets himself? To be able to shoot another player, you have to intend to. This would not be visible to anyone else, would be able to be turned on instantly, but take longer to turn off (however long is fair). You can still be killed by other players whether this flag is on or off, but if you murder someone who did not intend to harm anyone, then whatever the ramnifications of PKing fall on you. However, if you kill someone who has had their flag on LONGER than you have, then the game could consider it self-defense.
