I havent seen the show, so I dont know who the characters are, but from what I understand the main character in this isnt a "good" character.
So from that, I can tell you that Light (i think thats who the main character is), is both the Main character and the Villain. If he has evil intentions, then he is the Villain. He also appears to be the main character. Both these roles can be fulfilled by one character. If Light's enemy is L (or whatever), who is a good character, L cannot be the Villain. Even if you look at it through Light's point of view (where what he is doing is for his own justice, and the other man is opposing his way to justice), the "good" character still CANOT be called a "Villain", because that would contradict his own motives (a villain being a character with evil intent). So L would be the Antagonist to Light, the Main character AND Villain.