"You don't have to be a gun! You can choose who you want to be!" -Hogart Hughes from the Iron Giant

And who did the Iron Giant choose to be? He chose to be Superman and save the world. The movie isn't clear about where the Iron Giant came from or who made him. It kind of implies that he was made by the Russians for evil purposes but the Iron Giant foiled their evil plot by choosing to do good instead with the love of Hogart who taught him what is good.