What does 2 Samuel 4:8 mean?

They brought Ish-bosheth's head to David at Hebron and said, 'Here is the head of Ish-bosheth, Saul's son, your enemy who tried to kill you. Today the Lord has given you revenge on Saul and his family.' 2 Samuel 4:8 - Modern Text Bible

(The brothers present the head to David, expecting a reward. They frame their act as God's justice for David, assuming he'll be pleased.)