He must pay for that lamb four times over, because he did such a thing and had no pity

 2 Samuel 12:1-15a

David burned with anger against the man and said to Nathan, “As surely as the Lord lives, the man who did this must die! He must pay for that lamb four times over, because he did such a thing and had no pity.”

Then Nathan said to David, “You are the man! This is what the Lord, the God of Israel, says: ‘I anointed you king over Israel, and I delivered you from the hand of Saul. I gave your master’s house to you, and your master’s wives into your arms. I gave you all Israel and Judah. And if all this had been too little, I would have given you even more. Why did you despise the word of the Lord by doing what is evil in his eyes? You struck down Uriah the Hittite with the sword and took his wife to be your own. You killed him with the sword of the Ammonites. Now, therefore, the sword will never depart from your house, because you despised me and took the wife of Uriah the Hittite to be your own.’

“This is what the Lord says: ‘Out of your own household I am going to bring calamity on you. Before your very eyes I will take your wives and give them to one who is close to you, and he will sleep with your wives in broad daylight.  You did it in secret, but I will do this thing in broad daylight before all Israel.

After Nathan had gone home, the Lord struck the child that Uriah’s wife had borne to David, and he became ill.

Why?

Nathan tells a story to show David's sin.

David killed Uriah in secret and got his wife, the punishment is the sword will never depart from David's home, and there is calamity for David and his home. David's wife will sleep with other men in broad daylight.

Nothing can hide from the Lord. Our God knows everything.

David has mentioned what he has to pay for he took the wife of Uriah and killed Uriah. Four times payment.

How?

Nothing can hide from the Lord, everyone has to pay for what he has done. For God is righteousness forever.