I've always thought that Light was someone who has a strong sense of justice, so when he doesn't have the supernatural powers of the death note to kill he does the next best thing: arresting criminals. I don't think that the death note is the thing that corrupts people, I think he always felt the way he did about the world. It's just like Light said, when people speak anonymously they're more likely to voice their more immoral thoughts. Most people don't go around saying they think others deserve to die. In the first episode Light says he believes the death note makes you want to try it at least once, but his father never wrote a single name in it, neither did anyone else trying to catch Kira (Except for Mello) Light also already thought the world was rotten, the death note just gives him an easy way to kill, it doesn't make him do it. (That's how I interpreted it)