Wasn't Joab only doing what the Law allowed an avenger of blood to do?

Here is what the Law and these chapters say. The Law gave the avenger of blood a part: "The avenger of blood shall himself put the murderer to death" (Numbers 35:19). It also set apart cities of refuge, "that the man slayer not die until he stands before the congregation for judgment" (Numbers 35:12), and Hebron was one of them (Joshua 20:7). 2 Samuel 3 doesn't use the Law's words here. It doesn't call Joab an avenger or Abner a murderer, and it doesn't mention refuge or a judgment. It gives Joab's reason, "for the blood of Asahel his brother" (3:27), and says Asahel was killed "at Gibeon in the battle" (3:30), after Abner had twice told him to turn away (2:21-22). It gives David's words: "May the LORD reward the evildoer according to his wickedness" (3:39). Near the end of his life David tells Solomon that Joab killed Abner and Amasa "and shed the blood of war in peace" (1 Kings 2:5), and Solomon speaks of "the blood, which Joab shed without cause" (1 Kings 2:31).