He didn't need to do it, and despite him saying that it wasn't 'redemption' but 'desperation', he obviously feels strongly about it and wants to do at least some good before the end. It seems to me that after this level Max starts to reevaluate his own views of life, and in the next chapter where he destroys the organ harvesting ring he gets shocked into actually going out of his way to helping the world around him in some way, which wasn't really his motivation in the first two games. Passos' last words to Max are 'life is worth living', and Max doesn't really know what to say. Not only did he destroy the organ harvesting ring and take down a few corrupt cops and politicians, but he also saved Giovanna and her unborn baby and reunited her with Passos, ensuring they'd go off and live a good, normal life - giving a troubled cop and his family a chance to live the life that Max never got to. He went to hell and back to stop the evil scumbags and actually made a positive different in the end.
Some people say that Max is still depressed and hasn't moved on from his past, but I didn't get that impression. I just finished my third or fourth replay of the game (hadn't played it in a few years though), and my impression was that it was actually pretty optimistic (for this series' standards, that is).
