[4] No killing somebody over and over without provocation
Grief killing (killing somebody multiple times for no new real reason) is frowned upon. You can kill anyone you like in game, for any reason you want (characters are allowed to be douches), but doing it multiple times without a new reason each additional time is against the rules. This means that you cannot kill a character 3 times in a row because they attacked you once. Each kill requires a new provocation for it to be legal.
Decent reasons for killing a character would include retaliation for being attacked, in defense of an ally, in response to a serious roleplay insult, bounties, etc. You may kill a character for any other reason (or no reason at all), but doing it more than once in a row without provocation is considered bad form, and only serves to drive people out of the game.
The one exception to this rule is in the case of faction VS faction warfare. Where peace isn't made, the enemy doesn't have to wait to be hit before hitting back.
What about in light of the Offline AI system in which you can set it to automatically attack enemies upon sight, and might continue to attack / kill someone beyond the first altercation? Were is the line drawn?