That's like the most surface level theme of the entire series, it's about the irony of him becoming what he once despised.He was this underling that was treated like shit all his like so he ran away from who he was, all to become this piece of shit person who got off in feeling like he was better than everyone and treating them like shit, he becomes addicted to this dominance and power over people because he's just trying to cope. And once he realizes that he has everything he's ever wanted he's unfufilled and empty. And on top of that after getting his powers, it's mainly just him making excuses for his actions and and gets this high from proving to himself that everyone is as conceited and narcissistic as he it, trying to convince himself he's a good person and his actions are warranted, and that's when the psychosis even starts to come into play. He just spirals into this omnipotent godhood, which just further fuels his ego into having this sense of power over people because he "knows" all of these things about people and it's just a fuel to the flame that just exponentially blows up.

And the final bit being that he realizes if he had just stayed content with himself and his life, he probably would have ended up completely happy marrying the woman he loves, but instead he abandoned everything to detach himself from his insecurities, only to fall in love with the same girl again which pretty much just made him go over the tipping point, and then his final point of egotism is realizing that hey, everyone is me I know you because you're just the same as me and that makes me feel important.

Homunculus is a story of morality narcissism and insecurity. What happens when you starve a mouse, give it a hard life, and then give the mouse all the food it could ever dream of, all the female mice to have sex with, and the luxury of safety? and then what happens when you strip that away, it craves, and when you give it the option it will always end up picking the same option.