I don’t really see why this is a question to begin with. It’s clearly stated that there is indeed a real reason why the Cabal are doing this. Due to the failure of the Sundial mission, the Red Legion has no more options left, no more plans, no more cards to play. Launching the Almighty is their last move of pure desperation. Rasputin has nothing to do with this. The Red Legion isn’t stupid, they know who and what he is. They wouldn’t listen to him if he told them to do something so crazy. They definitely no he is their enemy. Additionally, how would he even communicate with the Cabal, let alone without someone like Ana catching wind of it? And that cutscene with Osiris and Rasputin was clearly placed in the context and timeframe of Season of Dawn. As far as I remember, Bungie has never shown us a cutscene that takes place in the future, especially not that far. And lastly, as others have already said, Rasputin has never stopped protecting humanity after his reemergence. He clearly states in the end cutscene of Warmind that this is still his primary directive. He’s distrusting of Guardians and the Traveler. His method of protecting humanity are unorthodox to us and have been controversial, but he would never let the Last City get wiped out like that.