Raito also Know as Light Yagami found himself in a situation of "justice". That soon became a killer intention rather than a lawfull persuit.
After the first murder, the second or third come easily for a killer. As things run it's course he makes the decision, that is, to cleanse the world, he quickly transforms into another person, killing over 390 people in chapter one alone. Killers who never get caught almost always hide themselves within society and think ahead of the game, as Raito does. I strongly believe that the change was crystal clear, to show a real killer’s transformation through Raito’s use of the Death Note first from an innocent boy, then to one who kills only the "bad people" and finally into a mass-murderer who is willing to KILL anyone who gets in his way (including his family, and those who did not commit a crime at all- like L).
So many times, he states that he will rule over a world where crime will not exist. Looking more closely into this statement, we notice the most important word of all “he will rule”. Raito wants sole supremacy—everyone around him is just a mere pawn in his game, and if he desires justice so dearly, why does he kill off those trying to seek justice as well?