When Wonder Woman solos the JLA with her skills it is called respect. When Hawkman KOs Superman it is called respect. The idea that Batman with years of prep and one of the largest businesses in the world combined with access to extraterrestrial technologies could legitimately take down the league is blatant fanboy ism and PIS to a swath of the community.
Each of the core members has the resources to take out the other high tier members if it comes down to it. Firestorm, Captain Atom, Metamorpho, Zatanna, Dr Fate, and prep Constantine could take down the league of need be.
MM if he goes all out supes if he sun dips phantom stranger if he REALLY wanted 2 batman wit 5yr of prep aquaman wit da entire atlantean army WW wit da entire amazon army green lantern wit full ion power & flash using 100% of da speed force
If the character can prep, has the element of surprise, etc, then a good chunk of them could do some serious damage. They could probably take out at least 5-10 heroes before anyone even begins to suspect a betrayal. Or, they could just subtly make mistakes, fail to protect team mates etc that screw over team mates during missions against villains.