Light Yagami started out with an overinflated ego and impaired self control by virtue of being a teenager with good grades. He likely was also already a psychopath (though you can’t give that diagnosis to a minor). He’s effectively the male Japanese version of Elizabeth Holmes. That’s what led to him becoming a megalomaniac.

Should be noted how Misa Amane isn't really corrupted, just gulled into Light's cult of celebrity and used up by his machinations.

I think a few more characters with a hand on the Death Note would have made the story better, to play out it's influence on the user.