Ishbosheth found out that Abner had been with his father Saul’s concubine, Rizpah. He asked Abner why he did it. Abner was offended that Ishbosheth confronted him, and vowed to help David overthrow the house of Saul. Abner sent messengers to David offering to help him win the kingdom. David accepted his offer on the condition that he bring Michal, Saul’s daughter and David’s wife, to him. David sent to Ishbosheth to have her sent, and he took her from her new husband, Paltiel, and delivered her to Abner. Abner began to encourage Israel to accept David as king, and brother Michal to David at Hebron, where they had a feast.
David sent Abner away to bring Israel to him, and Joab came to David, asking why Abner had come and gone in peace. He said that Abner was deceitful and not loyal. He sent for Abner, and when he came back, Joab stabbed him in the stomach in revenge for his brother Asahel. When David found out, he disavowed Joab, and declared that his household be stricken with leprosy or other skin diseases.
Revenge is a strong motivation, but it is not the proper response. In this case, God’s will was being aided by the actions taken by Abner. When Joab killed Abner in revenge for his brother’s death, it had the potential to get in the way of David’s kingship. David saw this as a betrayal on Joab’s part, and cursed him for it. Don’t let revenge motivate you to get in the way of God’s plan!