Light...the mind of a kira
Throughout our lives, there are many occasions in which we find ourselves asking: do the ends justify the means? That question is posed throughout the duration of the anime and manga Death Note and that question is allowed to become more specific throughout the themes of the show.
The questions posed by Death Note is this: should we be allowed to decide the fate of others as judge, jury, and executioner just because we find their crimes heinous? Furthermore, when we lose interest in these ideals of morality, should we be given the freedom to rewrite them all together to sculpt what we believe to be a better world?
For Light Yagami, all of these questions are a resounding "yes," and because of that, he feels no guilt for what he wants to do. After all, Death Note is a mystery/action thriller anime and manga about a young man who is given the power to kill others by way of a Shinigami’s death notebook and decides to kill criminals through his own single-minded brand of justice.
I am Justice!
But what motivates him to do it? A true sense of decency and justice? Or is it fair to accept the truth—that he was bored with the way things were. As a whole, Light was born with exponentially more luck than anyone else.
Both of his parents are alive and well. He has a cheerful, healthy sister, and he’s a high honor student, one of the smartest in the Kanto region of Japan, in fact. Beyond that, he is the comfortable son of a police chief, which gave him a strong moral stance. But because of all of this, to Light, his life is just the same as everyone else’s, leaving it feeling mundane and leaving Light feeling as if he ultimately has little worth in the world. Light is someone who wants to matter to others, for his very presence to affect the world in a meaningful way that gives him a sense of peace and purpose.
That need for purpose intersects with his need for justice; he doesn’t want the world around him filled with vicious and terrible things around every corner, destroying millions of innocent lives, all while he just exists unaffected from the sidelines.
...but who gives up life to wield death?
Light simply needs to be a savior in order to prove all of the respect and honor were given to him wasn’t misguided—that he is important, one of a kind. Truly, that he could be the only one who could actually change the world.
That’s why when he finally finds the means to make such a change thanks to Ryuk the Death Reaper, he takes the opportunity and runs. This opportunity comes in the form of Ryuk attempting to embrace his own sense of boredom with the way things work on Earth and dropping the most powerful murder weapon on Earth—the Death Note—just for fun, and right into Light's path.
Its power is unleashed upon the evil of the world through the hands of light, and Kira, a Japanese term for killer and Light's alias, gets a following. Here is where Light’s talents for misdirection and lies are put into play, and thanks to his police connections, he has a wider database on who to kill that makes him absolutely deadly.
Because of all the freedom, he’s given, his longing for change becomes a lust for power. Sure enough, his hidden darknesses surface, and his methods become more brutal and manipulative. But, in spite of this, his reputation continues to grow, and no matter what happens to his life, family, or soul in the process, it no longer matters to him, because all that has happened and may continue to happen at his hands will always be... "just as planned."