Yes at first he only punishes criminals, but he ends up trying to proclaim himself as a god, which in itself arguably constitutes an evil person. Then he kills not only what
he decides as evil people, but anyone who opposes him. He also uses Misa for his own ends, knowing she is going to shorten her lifespan, not once, but twice! This isn't a "gray area of morality" here. When you kill that many people who haven't done wrong to others, it's villainy.
Light is just a well written villain who happens to be the main character. Death Note could've easily been written from L and Near's perspectives, since they are the "heroes" of Death Note, but it would have been less interesting. That doesn't make Light any less of a villain, even though he is the protagonist. I use the same reasoning Wikipedia does,
However, it is entirely possible for a story's protagonist to clearly be the villain of the piece, as is evident with characters like Vic Mackey (from The Shield), Tony Soprano (The Sopranos) and Walter White (Breaking Bad).
Non-Kira Light could be considered a "hero" I suppose.