PK ("Player Killer") guilds are focused on slaying other players' characters, often regardless of whether they are innocent or are themselves murderers. Generally, they have no reservations about the types of victims they will be killing, and some PK guilds will even murder members of their own race or those of their own faction, unless they happen to be in the same clan.
A PK's sole purpose is to PvP, and not to PvE except to obtain more equipment for PvP. Such clans are drawn to [Free For All] game like Darkfall, because such activity is actually accepted widely. When dealing with such, everyone is a target, especially if looting players is available as well.