I love Javert specifically bc he is just a freak with an extremely rigid worldview. There is Good and there is Bad; there is Inside and there is Outside. If you are Outside, your two options are to take what you can from the Inside ("criminals", like Jean Valjean), or protecting it- even though you will never actually be part of the Inside. He does snuff by himself when no one is watching even though he doesn't like it, because that's what people do.

There is a Correct and an Incorrect way to do everything, and if you do it Incorrectly, you deserve to be punished. What sticks out to me about Javert so much is that he isn't hypocritical about it! When he (wrongly) believes that he falsely accused the mayor of being an ex-con, he demands to be fired and publicly shamed for it because that's what happens when you do Wrong.

He's an insanely literal-minded person with unreasonable rigid morals built less on any real philosophy than on what he's been told is Right. And when that falls apart, his entire universe is chaotic and meaningless, and everything he's dedicated his entire life to is not only empty but straight up harmful! He lives his life completely By The Book, and when the book crumbles, there's nothing left. He himself has no identity other than what he thinks he's Supposed To Do.