If you dont save him, the game ends and the credits roll. You then hear Oracle sending a transmission for however hears it saying how Joker killed everyone, that he had won and there is not a single person that can stop him...it ends with her asking for help.
Then the game rewinds and sends you back to choose the correct way XD
Real Hulk: Gets stronger the more angry he gets. GameFAQs Hulk: Gets stronger the more angry his fanboys gets. And they are always super angry.