They aren't all as black and white as The Hulk-flipping from good to evil with a burst of anger. That line is often blurred in anti-heros, so why use the Hulk as an example?
In the movie, The Avengers, the Hulk is feared and rightly so. David Brenner can't control this beast. Once unleashed, he will destroy anything that gets in his way. We know this. So midway through the film, when they are airborne and the most vulnerable and the Hulk emerges, we know what's coming. But... and this is the but that makes him an anti-hero... we are always rooting for him, wishing just this once, the Hulk will change and become a decent guy. David Brenner's got to be in there somewhere, right?
Anti-heros are bad guys that have the potential to do good.