“So all the people and all Israel understood that day that it had not been the king's will to put to death Abner the son of Ner” (v37). Even in a tribal society, there were codes concerning killing and warfare. Abner, previously loyal to Ish-bosheth, had switched sides, and he approached David with promises to help him unite the kingdom. However, Joab heard this news and plotted to murder Abner as vengeance for the death of his brother Asahel (ch 2). David condemned the murder as out of bounds, shameful for a warrior (v29).
“The Lord repay the evildoer according to his wickedness!” (v39). We trust that in the end – even if earthly justice is imperfect – God’s righteousness will prevail.