Save the cheerleader

Future Hiro, is going back in time. Retracing all of the steps that have been taken and trying to change them - hence the huge timeline in Isaac's studio. He traced all the way back to the day that Mohinder got his fathers ashes, and went back in time to tell Peter to save the cheerleader, save the world - remember on the subway? Future Hiro continues to retrace the history that has occured, so that he can identify the times he needs to go back and tell someone to do something differently. He tells Peter to save the cheerleader so Sylar wont kill her and get her power of regeneration. He tells Now Hiro when he needs to kill sylar, because when he tried to do it in HIS past, he failed, because Sylar had killed Claire in his past and regenerated. So by changing all of these things in the past you can change the future, and eventually not have a world thats as messed up as the one that Now Hiro teleported to with Ando in the past episode. That episode serves as a what would/could happen if you dont change things. Get it?

Just pay attention to the next episode That is my suggestion

It was not accurately explained. The only thing that I can think of is that he stabbed Peter when he was trying to explode, thinking it was Sylar. The two really do look a lot alike. Another think I can think of is that when Hiro went to the future, the future changed with him. But, that is kinda complicated and doesn't explain a lot of things.