Ya'll conflate villain too much. Antagonist is what he is currently. Hes competing for One Piece, which is literally the definition of an antagonist. He's not "villain" territory yet, but he very well could be depending on his intent for Luffy and who he was discussing with the Gorosei
Nah, he'll never be a villain. I wouldn't even call him an antagonist, it's true that for now shanks is depicted as another pirate searching for the one piece but I still think that shanks knows luffy is the one who's going to be the next pirate king and has ever since luffy ate the gum gum fruit.
Roger likely learned about joyboy, nika, and some kind of prophecy on laughtale, told shanks about it and told him to help whoever ended up eating the df. That's why the rhp stole it in the first place when they have no interest in devilfruit themselves.
I've said for a while that the final race for the one piece will likely be between the strawhats, rhp, and Buggy. It will be a competition between friends.
Bb will be dead before that happens