Get the last entity damaged another entity.

[Bukkit] EntityKiller.java

public static Entity getLastEntityDamager(Entity entity) {
	EntityDamageEvent event = entity.getLastDamageCause();
	if (event != null && !event.isCancelled() && (event instanceof EntityDamageByEntityEvent)) {
		Entity damager = ((EntityDamageByEntityEvent) event).getDamager();
		if (damager instanceof Projectile) {
			Object shooter = ((Projectile) damager).getShooter();
			if (shooter != null && (shooter instanceof Entity)) return (Entity) shooter;
		}

		// Add other special cases if necessary
		return damager;
	}

	return null;
}