It's not a question of rights to self defense. Do they have the right to counterattack and kill people that were not involved in the fighting, in response to similar actions against them?
Counterattacking is not defense, though. It's attack.
edit: Thinking about this further, I would classify defense as any activity that degrades an attackers ability to hurt you. If someone throws a punch at you and you break their arm, they can no longer hurt you with that arm. Your "attack" was a form of "defense".
However, when your attack does nothing to degrade the enemy's ability to hurt you, is it defense? I would argue no. It's something else, perhaps revenge. Which is not defense.