Just because he didn't say he'd kill innocent people from the get go, he went from being a fairly high achieving genius kid who seemed completely normal to full blown psychopath willing to kill pretty much any criminal to make the world a "better place." The amount of hubris that takes alone is terrifying and his willingness to kill anyone that readily is just a glimpse at how little he values human life if he doesn't approve of what they do.

The guy was a full blown psychopath who only needed a little taste of power to set him loose. Like I said by the end of the first episode it is abundantly clear that he has lost it and isn't just headed down a dark road: he started out already on that road and hopped a bullet train to pitch black crazy bastard town. It was clear from the get go that he wasn't a hero, and the entertaining back and forth chess game with L is the entertaining part.

But I was never actually rooting for the guy other than in my usual root for the villain because I like a good villain way. He was a horrible person from the start, became worse as time went on, and got a better end than he really deserved when it was all said and done. For the things he did and the number of people he killed he deserved a much more painful end.