@EventHandler
public void onDeath(PlayerDeathEvent e){
   Player player = e.getPlayer();
   Entity killerEntity = player.getKiller();
   if(killerEntity == null){
      // Killed by nothing
      return; // Stop code
   }
   if(killerEntity instanceof Zombie){
      Zombie zombie = (Zombie) killerEntity;
      // Killed by zombie
   }else if(killerEntity instanceof Player){
      Player killer = (Player) killerEntity;
      if(killer == player){
         // killed himself (arrows?)
      }else{
         // killed by player
      }

   }
}