Joab couldn’t believe it. He refused to. He held onto the bitterness he felt toward Abner. He was ate up with hatred for the man. Instead of following after David by choosing to find forgiveness he killed Abner.
David was distraught. Regardless of the treason Abner had committed he was still his son.
Maybe that’s why David was able to forgive him so easily.
And if Joab would have put himself in David’s shoes perhaps he would have been able to forgive also. At least he would probably have refrained from killing his king’s son.
If you’re having a hard time dealing with an issue try to look at the situation from several different points of view. It may not change your outlook but it could help you not to react so drastically.