I think it’s the fact that Aziraphale doesn’t actually want Crowley to change, not really, that kills me. He wants to protect him, make it easier for them to be together. He doesn’t want to be separated from him any more than Crowley wants to be. He still wants to be good and he thinks that in taking Crowley with him to Heaven they can still have the same relationship as they do on Earth. Aziraphale doesn’t understand, not like Crowley does. Crowley has been at the mercy of both Heaven and Hell, quite literally punished by both. Aziraphale went to Hell to save Crowley and witnessed the blunt cruelty of Hell. Aziraphale still believes Heaven to be pure because Heaven covers its corruption with ‘good’ intentions. Crowley knows better.