I'm a bit confused, too.
But my guess is that Peter couldn't control all the powers he got being around Sylar, and he exploded. I don't think Sylar was ever the bomb.
Future Hiro thought the key was to "save the cheerleader, save the world", but that wasn't enough - which is clear from what we saw last episode. I'm thinking that in the altered timeline, Sylar didn't get stabbed... maybe Hiro had to learn that he was supposed to stab him at a point before Sylar meets Peter to prevent Peter from exploding?
Sara
