That’s his business (1 Samuel 26)

“No!” David said. “Don’t kill him. For who can remain innocent after attacking the LORD’s anointed one? Surely the LORD will strike Saul down someday, or he will die of old age or in battle.

It is the LORD, and only the LORD, who is the ultimate judge. And whether He chooses to judge Saul now in this life, or then in the next —that’s His business.