2 Samuel 3:6-4:3; 2 Samuel 4:5-12 (New King James Version)
Abner Joins Forces with DavidAbner was accused by Isbosheth (Saul's son that had been appointed king of Israel) of "going in" to his fathers (Saul) concubine. It never says it is true, but Abner got mad at him so much so that he was afraid for his life. Abner then send a messenger to David that he would align himself and the kingdom of Israel with him and David said he should come, but to bring his wife Michal (Saul's daughter) with him. Michal had been promised to David a long time ago, but Saul had given her to another man. When Abner takes her away, he follows crying in grief until Abner tells him to go home.
So, David took the kingdom and his wife too.
Joab Murders AbnerWow, Joab kills Abner because Abner had killed his brother Asahel in a different battle. David said that his kingdom was guiltless of this and that in Joab's house forever there would be someone that was diseased because of what he had done.
David’s Mourning for Abner
Ishbosheth Is MurderedEwwwww. Two men go murder Ishbosheth and cut off his head and take it to David. David tells them that the man who reported Saul's death thought it was a good thing and expected a reward. His men killed him. Why would these two men think that they had done a good thing by murdering Ishbosheth, Saul's son? He has his young men kill these two, cut off their hands and feet, and hang them up for all to see.
Our God reigns. Our God judges sin. Our God is an awesome God.