2 Samuel 4:8
And they brought the head of Ish-bosheth unto David to Hebron, and said to the king, Behold the head of Ish-bosheth the son of Saul thine enemy, which sought thy life; and the LORD hath avenged my lord the king this day of Saul, and of his seed.
The two assassins carried the severed head of Ish-bosheth to David at Hebron and presented it as proof that vengeance had been taken that very day against Saul and his descendants. They described Saul and his house as David's enemies who had sought his life, and they framed the murder as the Lord's avenging of David.
Their offering was a treacherous bid to win David's favor, and dressing the deed in language of divine vengeance did not make the crime acceptable. Treachery of that kind was not a rightful means of establishing David's rule. God Himself had established David's throne and delivered him from his adversaries, so David needed no such bloody act to secure his kingship.
David condemned the men for striking down a righteous man in his own house and in his own bed. He punished them for the murder rather than rewarding them for it.