Future Hiro couldn't go back to kill Sylar before he got his powers because he doesn't know who Sylar really is. The name "Sylar" is an alias. Only Bennet, "the company", and the late Dr. Suresh know his given name, Gabriel Gray. The only times when Hiro knew he could find Sylar were ground zero of the "bomb" and Claire's murder. He chose the earliest of those.