I think "snitch" is just a pretty poor adjective to describe what a lot of people might think is going on with Shanks. When 907 happened, I didn't think Shanks was part of the WG, I thought there is just simply a lot more complicated depth when it comes to his character.
Shanks is by far the oldest character in the series save for Luffy. With the few breadcrumbs left by the series regarding his character and possible direction, you CAN interpret his character to be far more menacing than initially entailed.
@ArturCantSpeakJapanese has a pretty good writup on this, but there is a decent enough reason to believe that Shanks may be the true leader of the Underworld. Think about things that tie the other Yonko together. They've all been described as mob bosses in their own way, controlling a syndicate that networks it's way around the new world. BM controls islands outside of Totland via her "protection", in payment for sweets and food. Kaido ran the smile and weapons business with Doflamingo and has taken over Wano. Blackbeard is an all controlling tyrant who turned on Whitebeards protection status to pillage those islands for everything they had (see Chapter 904 with the Revo commanders)
So where does that leave Shanks? A man who's past is unknown? A man who's motivation to meet Luffy is unknown? A man who's reasoning for meeting the Gorosei is unknown? What "trait" of control does he carry? Clearly he uses his symbol for peace (see Barto's cover story where he burned his flag lol), but at what cost? Shanks is very clearly a violent person. He took Kid's arm off like it was nothing. Sure he's stopped Kaido and Blackbeard, but that doesn't require him to be all good. Luffy has 100% the intention to beat him, but in what capacity?
3 interesting things I'll leave from
@ArturCantSpeakJapanese 's points that are easy to understand why this might be the case:
- Shanks' bounty is a direct reference to Oicho-Kabu, a card game mainly played by Yakuza in history
- Shanks has only ever been referred to as Kashira or Boss, and not once as Captain. This is a direct Yakuza reference
- Yakuza typically bet on their fingers when they lose/make bets and have nothing to give for it. Shanks "bet" his entire arm on Luffy. For what purpose?
There is a strong reason to believe that what Shanks wants from Luffy isn't what Luffy wants. And THAT might be where the antagonism comes into play. It's not that Shanks is inherently bad, his goals might actually be really good/noble, but its the method of how he does things (secretly) that may spell out danger for the crew. His secret meeting with the Gorosei about 1 pirate is already an application of this idea, otherwise there would have been virtually no reason for Oda to draw it.
