A bit silly, perhaps, but reasonably in character.
Kas isn't a perfectly rational actor, he's an evil, duplicitous, backstabbing bastard out for revenge against his former lover partner-in-villainy. Even setting aside the specifics of his plans to usurp Vecna's ritual for himself, Kas would never have been satisfied with just handing over everything he knew to Mordenkainen et al. and letting them take it from there. However it was accomplished, he'd want to be personally responsible for bringing Vecna to ruin.
The Wizards Three were never going to be anything more than tools to advance his own plans, as far as Kas was concerned.