When you die, it's not yours any more.
Edit:
It's not even this. This is an MMO. When an item is in your inventory, it is yours. When you expend that item in order to spawn a monster, the claim on that monster is yours. When you lose that claim, it is no longer "Your" monster. It is a monster, with no claim.
Spamming cure on a tank and forcefully removing claim from another group? That's stealing.
Taking an unclaimed monster? That's just normal. You're not taking it from anyone at all. No one owns it, as the original owner lost their right to it when they lost claim.
There is no victim. There is no theft.



