Sylar killed Arthur Petrelli. Arthur had all of Peter's abilities, plus who knows how many other abilities. Yet Sylar, who had gotten his hunger back, his lust for power, killed Arthur...
...and just walked away. What? Peter couldn't do anything to Sylar. Haitian was drained from trying to hold Arthur back and probably needed to rest for a bit. At the very least Sylar could have TKd himself and Arthur out the window and gotten away, giving himself plenty of time to take every last power Arthur had.
But he didn't.