Him being against Luffy for his own purpose as a competitor for One Piece, does not mean he's evil. I 100% believe he will be against the WG as well. Same as Blackbeard. Same as the Revo's. They all have their own aligned goals.
In Vol 4 Billion, Oda already alluded to Shanks having his own goals about the new era. Whether or not this aligns with Luffy is the real question. Ofc he's against the Marines. The question is, how deep does it go with the Gorosei too, even if he is against them?
It might also be due to the fact he was threatened with a 100 death threats by text, from the wife of a former assistant of his. I get the feeling that made him more reserved. Some mangaka seem a lot more discreet these days, hiding their faces and such.