NPCDamager.cs

using UnityEngine;

public class NPCDamager : MonoBehaviour
{
	private void Update()
	{
		if (Input.GetButtonDown("Fire1"))
		{
			DamageNPCs();
		}
	}

	private void DamageNPCs()
	{
		foreach (var npc in FindObjectsOfType<NPC>())
		{
			npc.TakeDamage(5);
		}
	}
}