In one piece you can be an enemy to the MC and not be a villain. Lets go over all of the cases why this works for Shanks:
1. Luffy intended to fight Ace, his brother, for One Piece
2. Shanks intends to meet Luffy at his strongest:
3. Luffy wants to not only surpass Shanks, but physically beat him (lets stop acting like Big Mom has the same narrative importance as Shanks, its embarrassing people bring it up like it means anything):
4. Shanks looks at Luffy's bounty poster ominously prior to becoming a Yonko (why does this seem to happen virtually every single time too?):
5. Shanks meeting the Gorosei is reason #1 why we should all be doubting his character, good or bad. The scene is a contradiction, as Shanks, protector of the weak, is discussing issues about a "certain pirate" with people who literally enslave the weak and are very public about it:
6. Shanks, savior of the weak and fair fights, goes to Wano to ensure Blackbeard doesnt attack Luffy here, but in the same breath, decides to go after Luffy's fleet responsible for burning his flags:
7. Shanks also decides, once again ominously, to go for One Piece now, a direct competition against Luffy and Blackbeard:
8. Then while giving Kid a chance to leave, quite literally DESTROYS him when Kid does the opposire and we are left with yet ANOTHER ominous image of Shanks in the end:
9. For fun, you have this comparison between possible "traitors" where Oda only seems to draw people like this when they intend to betray someone in the future. Note that Apoo and Kanjuro's images come hundreds of chapters before they were shown to have betrayed their respectivr groups:
Just food for thought. I think people have this BB vs Shanks thing WAY too simply made. Its more complicated than this, this literally is not all coincidence. There is definitely intent with how Oda draws him and certain key dialogue and scene posed by his character.
I also can see this tying hugely back to chapter 1 from a betrayal of ideals standpoint, and it is very much in line with Oda's "humor" on making his quote unquote "self insert" the main obstacle Luffy has in this story.
Just my 2 cents....I know the brigade will attack me for it