Being a tragic villain is not about not being a monster, it is more about suffering for one owns monstrosity while being oblivious to it (or in denial).
A tragic villain is someone who’s the cause of one’s own misery, and nonetheless keeps doing the same horrible acts that amplify the pain, thinking that is the course of action that will eventually deliver them from it.
It is someone that might accuse some external enemies of what is the mere byproduct of their warped state of mind, and commit atrocities in the name of their desire to be boundless, untouchable and secure.
They’re tormented, but they’re guilty for what they do, not only victims of a cruel worlds, but the perpetrators of that very cruelty. Moreover, what characterise a tragic villain is the impossibility of redemption, because they’re far too proud for that, far too gone, and convinced that their reasons are more important than anything else. Deep down they aren’t entirely devoid of conscience, desire for connection and love: they just choose to ignore them, out of pride (and fear).
Unfortunately Naruto gave many people a very warped vision of what a tragic character is: it is not someone who’s deep down is a good person, driven towards the wrong path by some terrible event in their past, but someone who turns their life into a tragedy.
I don’t think we will ever see such a villain in One Piece though, Oda doesn’t seem to be capable of portraying real moral complexity.