“But that is the truth of this shallow world,” he said. “All members of the Golden cabal will assume that Aleister Crowley, a mere pawn, occupied the Blythe Road ceremonial ground on the official orders of Mathers. In order to take everything away from the Westcott faction. …It’s a strange thing, Mathers. They will think that even though you do not recall signing such an order.”

“You can’t mean…”

“Yes, yes. Did you think I made a nearly suicidal attack on you for no reason? I wanted your blood. With a single drop, forging a document was quite simple. In fact…didn’t Westcott forge a letter from Anna Sprengel to give the organization more prestige when creating the cabal?”

[...]

“He never expected the two sides to entirely wipe each other out. One side was sure to win and the survivors would take over. …All he wanted to cause was chaos. And then he could slip into the confusion to sneak in close enough to stab both sides before they saw him coming. To the end, Crowley only ever thought of fulfilling his revenge by his own hand.”
He was cunning.

[...]

When he killed a Westcott faction magician in a back alley, he carved traces of the Mathers faction into the corpse before dumping it on the main road.

When he killed a Mathers faction magician in an underground tunnel, he placed a Westcott faction weapon next to it before leaving.

Toaru Majutsu no Index New Testament vol.18 ch.3 pt.13