public void onEntityDeath(EntityDeathEvent event) {

        if (event.getLastDamageCause() instanceof EntityDamageByEntityEvent) {
            EntityDamageByEntityEvent e = (EntityDamageByEntityEvent) event.getLastDamageCause;
            if (e.getDamager() instanceof Player && event.getEntity() instanceof Player) {
                Entity victim = (Player) event.getEntity();
                Entity offender = (Player e.getDamager();

                //work
            }
        }

        ...

    }