NPC.cs

using System;
using UnityEngine;

public class NPC : MonoBehaviour
{
	[SerializeField]
	private int _startingHp = 100;
	[SerializeField]
	private UnityEngine.UI.Slider _hpBarSlider;

	private int _currentHp;

	private void Start()
	{
		_currentHp = _startingHp;
	}

	internal void TakeDamage(int amount)
	{
		if (amount <= 0)
			throw new ArgumentOutOfRangeException("Invalid Damage amount specified: " + amount);

		_currentHp -= amount;

		UpdateUI();

		if (_currentHp <= 0)
			Die();
	}
	
	private void UpdateUI()
	{
		var currentHpPct = (float)_currentHp / (float)_startingHp;

		_hpBarSlider.value = currentHpPct;
	}

	private void Die()
	{
		GameObject.Destroy(this.gameObject);
	}
}