So Ish-bosheth heard about Abner and lost his courage. So, it seems like Ish-bosheth even had courage when Abner betrayed him...but Israel was affected by it as well.
Dear Lord...Saul's son's men (I think) murdered Ish-bosheth, cut off his head and carried it to David allegiance as proof of their to him now?
David pointed out to them that he had slayed the man who brought news of Saul's death, insinuating himself into the story. How much worse was it to murder a man in his bed and desecrate his body for personal gain. Plus, David refers to him as righteous. He may have been a feckless follower of Abner, but he did have a pop-culture claim to the throne.
So David have them dismembered and slaughtered.