Commentators explain that David is using a common-sense proverb to prove his innocence. The saying, 'wickedness proceeds from the wicked,' means that a person's actions reveal their true character. David's argument is simple: if he were truly the wicked conspirator Saul accused him of being, he would have acted wickedly and killed Saul when he had the chance. His refusal to do so is proof of his righteous heart.