At start-up, NORMAL (can be attacked by hardcore) or HARDCORE check box. Hardcore = PVP enabled (can attack other players and only die once (without the help of another player to get resurrected).
Would that be simple to grasp the concept, and efficient to deter 'abusers' (PITAs).
Hardcore mode (sudden death) really isn't appropriate for a game like this, imho. Role-player or not, PvPer or not, having the character you could've spent multiple hours leveling vanish in a second is not something you want your players to risk going through. And normal players could only run away from PKers? Both options cause frustration, and tarnishes the overall gaming experience.
From a game-dev perspective, I'd rather have no PvP at all than something like this, where a few hardcore players could effectively PvP all the time and prevent anyone from building their own PvP characters to fight back. And with the option of subscribing for an XP boost and perks, you've got yourself a game-killing pay2win system.