to be honest, in a social group, if that actually happened it shows how shit your friends can be. If the gm okd it and a player stabbed your character in the back then i hope he gets expunged. Though i guess it would be the dms fault if he didnt explain it properly in the first place. But regardless, if players dont get a chance to play something interesting once in a while then why bother playing?
It all comes down to "fun"
And what you find "fun" I might find boring (and the reverse is also true)
Some people's "fun" is playing in a group and having a great time as a group...
Some people's "fun" is playing in a group, then doing everything they can to tear that group down.
Some people's "fun" is to actively and aggressively attack other player's characters in game (hurting, stabbing, killing, setting up the character to take a fall, Or some other action that hurts an other character)
Not to mention that all tables have different "table" rules.... and sometimes there is no limit on what your character can do, if supported by RP (including killing other players)
Without knowing what type of table Lord plays at, we don't know if they have any "table" rules that stop PKing.....or any other type of character on character violence. (and believe me, it does happen! just look around the forums here, you will find dozens of posts all related to temper-tantrums from a character getting killed.)