I don’t think that Oda’s interested in making grey characters to begin with. He even has Bonney immediately saying “not really” after saying “Rob people,” and such.
The issue with the argument of grey characters (I'm not targeting you specifically) comes with the fact of thinking that because there are grey characters in a story, therefore the story is better because it reflect the reality better.
The problem with that statement comes in two parts:
- It ignores the fact that stories are most of the times not meant to be a depiction of reality but art products meant to enhance reality to create meaning and emotions. (this is why Oda creates very few "grey characters")
- It perpetuate the idea that good representations are representations where characters can do good but will also do bad things which is just wrong. Good representations are just good people.
The purpose of grey characters is to create more mysteries in the dynamic of the narration, but when you already have a good dynamic of mysteries or good overall characters like in One Piece, then grey characters are not a necessity anymore.