In chapter 4, we learn of the odd story of how Ishbosheth becomes worried and weakened when he hears that Abner is dead, and so two of his captains come into his house when he's resting in the middle of the day, and they kill him. They take his head and bring it to David, which really seems like an obviously stupid thing to do given the way David has reacted so far to people who take it in their heads to commit violence to help him. Indeed, David reminds these guys of the fate of the Amelekite who killed Saul, and then orders them killed and their bodies mutilated. As I said before, David is probably in part motivated by setting a precedent that regicide will not go unpunished in Israel.
So long as I didn't already comment on it, and this post is so small, the phrase "under the fifth rib" is, I imagine, a Bible-ism for "in the heart".