You Shall Not Kill - 2 Samuel 3-4.

2 Samuel 3:26-30 Joab then left David and sent messengers to catch up with Abner, asking him to return. They found him at the well of Sirah and brought him back, though David knew nothing about it. When Abner arrived back at Hebron, Joab took him aside at the gateway as if to speak with him privately. But then he stabbed Abner in the stomach and killed him in revenge for killing his brother Asahel. When David heard about it, he declared, “I vow by the Lord that I and my kingdom are forever innocent of this crime against Abner son of Ner. Joab and his family are the guilty ones. 

2 Samuel 4:6-8, 12 The doorkeeper, who had been sifting wheat, became drowsy and fell asleep. So Recab and Baanah slipped past her. They went into the house and found Ishbosheth sleeping on his bed. They struck and killed him and cut off his head. Then, taking his head with them, they fled across the Jordan Valley through the night. When they arrived at Hebron, they presented Ishbosheth’s head to David. “Look!” they exclaimed to the king. “Here is the head of Ishbosheth, the son of your enemy Saul who tried to kill you. Today the Lord has given my lord the king revenge on Saul and his entire family!” So David ordered his young men to kill them, and they did. They cut off their hands and feet and hung their bodies beside the pool in Hebron. Then they took Ishbosheth’s head and buried it in Abner’s tomb in Hebron.