Joe is an evil assassin. I beat up on Joe with a super axe which exploits his vulnerabilities. But, Joe is not playing at the moment. Fred, also an enemy of Joe, gets my axe and gives it to Tahren. The world reforms. I go straight to the very secluded mob with the axe and Joe is there trying to get the axe he cannot use, but somehow knew was there.
I might kill Joe afk. This is hypothetical, but I've seen many things like this. Some more dubious than others. Also, if someone runs around full-looting for no reason. Also if someone befriends people only to backstab and loot them when they're trying to kill some mob together. I know this can happen IC, but if you write a role that requires this, know that I may kill you afk. Also, if I can tell you're a dick in RL from interacting with your char, I might not flee when I don't see you doing anything in combat. If I've died to you and you talk smack while I'm still a ghost: maybe. If your taunts are borderline OOC: probably. If you do that after a 4-1 gankdown and full-loot me: definitely.
Keep in mind, it's not against the rules to kill someone afk. If I'm afk and you don't kill me, I consider it a favor. I wouldn't kill afk except in rare instances like the one's I've mentioned.