I think I'll post here. not that my words mean anything, but the question interests me, so I post.
My view is one taken from how the game mechanics work.
Currently (as game mechanics can and do and will change over time), say I engage a mob, then run around not hitting it. The lock out that prevents others from attacking it, exceeds, and my sole "right" to that particular mob instance is negated. They are free to attack it and "take control" of that mob instance.
This time out is based on damage delt. If I'm not doing damage, I'm not engaging the mob, therefore I'm not owner of the mob instance, therefore, I'm not attacking the mob, thus it is free for anyone to engage in combat.
By defenition, attacking the mob requires me to be hitting it. If I am not hitting it for a period of time, this lockout expires, and the mob becomes free for someone else to attack, even it if is still currently attacking me.
Therefore, standing there in full defense, not doing damage, not keeping the lockout active, means you are not attacking the mob, therefore it is free for anyone else to attack.
Killstealing, would be if one player is doing damage to a mob, keeping thier lockout active to other players, and another player comming along, using a bug exploit to stop the curent player from being able to do damge to the mob, and take away thier lockout of said mob.
Simply comming along and attacking a mob that is currently attacking a character, but the character is not doing damage back, whether it be becuase of being in full defense or simply becuase they are too low a level to deal damage back. If they aren't doing damage for extended periods of time, the lock out expires, and the mob instance becomes available for anyone to take.
This then is not killsteal by definition, becuase, the game mechanics itself, allow for any player to come by and attack it.
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another semi related topic: attacking and engaged in combat are not one of the same.
attacking is actively engaged with a mob with the intent to kill it/do harm to it. you are attacking a mob f you are doing damage.
engaged in combat encompases attacking in it, but they are not ne in the same since you can be in combat, but not attacking the target. full defense, you are in combt, as the mobis trying to hurt you, but you are not attacking the mob, as you have no intent to harm the mob since you are in full defense.
it's easy to confuse the two.
i hope this makes sense. writing posts after working 16 hour shifts and making them coherant is a bit hard.