The great thing about Dredd is that he's not a lunatic. He takes his work utterly seriously, what he does is sanctioned by the law, but he assumes utter responsibility for everything he does. He will kill the 500 million, but only to ensure the other 6500 million survive. If that means millions hate him, so be it, that's their privilege, though they should remember that it's only because of him that they're still alive to hate him.
He's like Control in "La Femme Nikita", an unmitigated, cold-blooded, manipulative bastard who plays god with human lives but it's made so much worse when you finally realise that A) He IS doing it to ensure the continued survival of the world and B) He's STILL better than the alternatives could ever be.