Light wouldn't be considered a totalitarian, he's not really deciding what is or isn't allowed in every aspect of people's lives. So far he's only judged people convicted of suspected of breaking established laws. He doesn't try to tell people how to live their lives or what to think, only what things they can't do. To start down the totalitarian path he'd have to start killing anyone who disagreed with his existance as this would constitute trying to forcibly stop people from behaving in ways that don't cause anyone harm other than him.