In One Piece, it's not as simple as heroes vs. villains, one clear-cut side vs another. This means that some characters like Sabo are friendly to Luffy, but ultimately owe their allegiance to other leaders or groups. Sabo may help Luffy sometimes, but he's got his own mission, so he's not about to join the Straw Hats.
Source：https://www.cbr.com/anime-characters-like-one-piece-sabo/
