Many people were convinced that Light felt justified at first in using the Death Note as he was only killing criminals, people he felt that the world would be far better off without, and alot of us at that point could agree with his logic. But then as with all things, power corrupts. The more he used it, the more his judgement became impaired, and he started to use it in revenge, and to further his own cause, to create the Utopia that he imagined, and one in which he would reign over as he felt he was the only one capable of doing so.