I thought about this issue as I read a post by K.W. Leslie this week, The analogy of the sloppy guard. The post is about a scene in I Kings 20:39-40, in which King Ahab orders a prophet (whom he thinks is a soldier) to pay money for a Syrian slave he had lost. The prophet's goal in bringing this fictitious case was to rebuke Ahab for not killing the Syrian king, Ben-hadad.
According to K.W., many commentators portray Ahab's order in a bad light, as if he's hard-harded and merciless. But K.W. says we should give Ahab the benefit of a doubt:
In part this comes out of that commentary I read. I really can’t get past the commentator’s immediate condemnation of Ahab for not being merciful. I get the feeling that it’s entirely based on a dislike for Ahab, or a presupposition that since Ahab is a bad guy—which he’s not—don’t give him the benefit of the doubt; everything he does will automatically be wrong, and it’s okay to condemn him automatically. Is that the proper attitude a Christian should ever have? Yes, people are sinner; yes, there’s such a thing as total depravity—where every inclination of a human is self-centered and sinful. But God calls us to be optimistic. God calls us to be merciful. We’re not to let Ahab’s sins slide, but we’re also not to judge Ahab as sinful without proper evidence.
As K.W. points out in other posts, there are times when Ahab at least tries to be righteous. He helps Elijah slaughter the prophets of Baal. He treats the Syrian god as powerless in a discussion with the Syrian king. Here's one I'd like to add: later, he has prophets of Yahweh in his court (I Kings 22). Granted, they're false prophets, but at least Ahab's gotten away from worshiping Baal!
And that's what I see in commentators' treatment of Joab. Some act as if he can do nothing right. They assume he must have some selfish, crass, or blood-thirsty motive behind everything that he does. And, indeed, David condemns Joab's bloodthirstiness (II Samuel 3:28-30; I Kings 2:5). But Joab did some good things. He was brave against overwhelming odds, resourceful, and put the ball in God's court to help out his army (II Samuel 10). He was loyal to David, sticking by him when few others did. Like Ahab, Joab was a mixture of good and bad.And so, from II Samuel 18 and what K.W. has to say about I Kings 20, I get a lesson about not judging people. Sure, there are times when people do bad things, and we should criticize them when they do so. But there are also times when we may assume that a person's motivation is bad when there's a possibility that it's not. In that case, why assume the worst?