Yoav's Ambush

After Yoav, David’s cousin and chief of staff, ambushes Avner, the late King Shaul’s defecting chief of staff, David launches into a dirge.  He curses Yoav’s family with the words, “May God repay the evildoer according to his evil.”  One interpretation could be that David thought Yoav might have been right to kill Avner, whose defection from his king and fellow Benjaminite to David, a Judean, might have been a ruse. If Yoav was wrong, he should be cursed, but if he was right, he did no evil and so there is no evil to be repaid.