Joab shows up quite a bit in 2 Samuel—and he doesn't exactly come out smelling like roses.
But to be fair, his actions are all pretty understandable. He's what you might call a "realist", someone who takes brutal views and a tough guy approach to problems because he or she thinks that's the only reasonable way to behave. That might not actually be realistic—but it's definitely a part of Joab's no-nonsense approach.
Joab seeks revenge against Abner, killing him for murdering his brother, and he executes Absalom against David's orders (though he makes some good points talking this over with David afterwards).
His most aggressive act might be when he treacherously guts and kills Amasa, who had replaced Joab as David's chief general. It's an extremely jealous and rather mean thing to do. (He also helps David carry out his treacherous plot to kill Uriah the Hittite.) Like David keeps saying about Joab and his brother, Abishai, they're simply "too violent."