What does 2 Samuel 4:7 mean?

They had come into the house while he was lying on his bed in his bedroom. After killing him, they cut off his head, took it with them, and traveled all night through the Jordan Valley. 2 Samuel 4:7 - Modern Text Bible

(They not only kill Ish-bosheth but also decapitate him, taking his head as proof of their deed, and flee under cover of darkness.)