I rewatched death note entirely (you prolly wouldn't like it since you are antiviolence a little) and after watching the anime long enough its easy to fall into the assumption that you root for the protagonist. But this is not the show that you are supposed to do that for. This show is like against the grain bad-assery so people love rooting for the underdog.
But the truth is his psychology is like almost a school shooter but not pushed over the edge yet, turned serial killer. I only know that because of this other video with strong language explaining somewhat the psychology in laymans terms with over-flourishing strong language (guy used to be a radio host so can be eloquent speedy and creative with his insults/criticisms lol) (if you want to see the video youtube 'rageaholic Faberge eggs' wont link it due to language) and its basically
rich spoiled kid told he is special all his life : no one treats them special at school : entitlement dictates they deserve more : they decide to act out.
Light Yagami in death note is more like
rich kid in a well off family who is a genius. He has boredom (studies show boredom is linked to crime, even for rich kids) he is a popular kid who is socially adept.
The cartoon doesn't show it directly at the start at all, but he always had sociopathic and psychopathic traits.
The first hint is when he gains ambition after first using the death note
I am a deep thinker, so this stuff is on my mind. When I was a Christian it was easier for me to root for Light Yagami, because that's the philosophy it teaches, The world is disgusting. Also, teen angst, its easier to see the negatives than positives. They say you never know what you love til its gone, like your family, but he had reasons to not notice, because 'the greater good' concepts. He threw everything of any value in his life away, then died alone by the end of the show. When I was a Christian I did not understand that. As an adult, and as an atheist its far more noticeable. As a teen Christian I was just rooting for the underdog really. Except, I loved L as a character, so eccentric n honest and logical and some non-social behavior. But Light Yagami was very good socially, its why he could take up lying so easily so quickly, and had alot more initiative than L, the true good guy in the story.
Near the end you know the author is not on Light Yagami's side. Because when Light thinks the world is basically his own, he says he is creating a 'perfect world' because it'll be crime free. But then it immediately cuts wordlessly to some factories giving off pollution on the other side of the river as if to show how its not perfect. Nothing fundamentally changed, and when Light dies, chaos may erupt knowing that violence will go unpunished for some time. The world will become normal after several decades, it'll only be a little more impactful than the cold war.
Which means he was playing this role for himself, and this ambition scene shows it. His entitlement complex. He thinks everyone is beneath him, because they are all easily lied to and stupid and he passes without trying for full scores on everything in Japan. He had no ambition, was just being a good son, til the death note. He had no purpose, til that.
Sorta like how rich kids might go and beat up hobos. Except he was also a genius due to solid up-bringing. Too bad he couldn't see what really mattered, or he could, but found his excuses to justify his actions. He had to do so, because his father is a cop, he was forced into being dishonest due to his life circumstance. Others won't notice that its because of his typical ideology in his family, his normalcy and passion for justice from his father, how easy it is to get away with stuff when you have an entirely unsuspicious of yourself family, an entirely non-critical family.
Others might think I am overthinking it, but if an artist made it this way intentionally, which they did, then its not overthinking it. It just would imply its too much for others to handle. I might turn this into a blog post.
When he specifically says what are you talking about, I am the best honor student in Japan, that's a sign he already had façade practice.
Narcissism is what he has in common to that type of school shooter. And he already knows he can trick everyone with appearances alone.