What he says is not actually true, Light Yagami uses his magical weapon of indiscriminate slaughter to murder all those who disagree with him. He has this deluded notion that he is a god of justice and goes about raising himself by slaughtering everyone he perceives as being a criminal - not that there is any judgment actually involved, anyone suspected of a crime is automatically guilty. And Light or his loyal thralls are exempt from this as well. Of course he also executes anybody who actually stands in his way. A lot of innocents get murdered for refusing to submit to the worship of Kira, notably world leaders, he also murders anybody investigating him that he cannot directly control. His objective all along was to be perceived as a god. And he achieves this through mass murder.

Hell, that makes him sound like he's Sarevok from Baldur's Gate.