Yoav (Joab) the Cryptic Character

Yoav (Joab) is, to me, the most interesting character I have read about in Tanach (Bible) yet.  You have your Avraham (Abraham), Moshe (Moses,) and David types, who, while they made mistakes, were clearly good.  Then you have your and Korach and Avimelech types, who were clearly bad.  But with Yoav (Joab,) while he has the wily intelligence and penetrating insight into human character helps him manipulate others to do his will, his own motivations are mysterious.  Did he assassinate Avner, and later Avshalom, against the orders of the king, out of personal vengeance or out of a sense of protecting the king from his own weakness?  He is fiercely loyal to David, but often directly violates his orders.  Most of his actions could be taken as either good or bad, depending on his motivations, which remain a mystery to us.  David, for his part, always curses Yoav, but states that the curse should be carried out by God, implying he himself is unsure of Yoav’s motives.