OBL put himself into combat on 9/11 so it did not matter whether he was picking his nose, brandishing a weapon, or sleeping when he was shot by a service member nearly 10 years later. For this op it appears he was declared a combatant (along with others in his entourage) by the highest national authority, therefore he could be targeted under the ROE being used. This is not assassination but a lawful military action. This was not a civilian law enforcement matter but an extraordinary military action, therefore neither police rules on self defense nor arrest warrants applied. Naturally every op has unintended consequences but that's why Obama is paid to be the "decider" ...