Based on what we seen so far, I think:
- Hiro is the cause of just about everything. He is the reason the world is doomed. He creates the time paradox that ultimately destroys the world. What causes it? Hiro originally thinks the world was destroyed when he first warps to the future, but in reality, it was just New York. Notice that future Hiro never leaves New York? He doesn’t know that the world was not destroyed and he keeps trying to fix things. At the end of the last episode (april 23) they show him creating timelines, which shows that Hiro himself is trying to sort everything out.
- What destroys New York? Syler explodes. This can happen one of two ways. Syler kills the nuclear guy and cannot control the new power OR Peter injects nuclear guy’s power into Syler and again, cannot control it.
