Albert Barnes Commentary 2 Samuel 4:12

"And David commanded his young men, and they slew them, and cut off their hands and their feet, and hanged them up beside the pool in Hebron. But they took the head of Ish-bosheth, and buried it in the grave of Abner in Hebron." — 2 Samuel 4:12 (ASV)

Cut off their hands ... - After they were dead. Their hands and feet were hung up in a public place, both to deter others and to let all Israel know that David was not involved in the murder of Ish-bosheth.