I love the delivery of this line.
You can hear the Aziraphale slip from his voice. You can hear the incredulity, the anger. You can see it in his face.
He's standing here watching the supposedly best-of-the best angels -- The Angels, archangels -- stand in a row like pearly white teeth in a callous smile whilst they calmly await the murder of the most good-hearted (but still a bit of a bastard) angel that he's ever had the opportunity to know. That is horrible, God awful. But it's not only that.
He's bearing witness to the killing of a fellow angel, whilst knowing he was yanked from Heaven like a rotten tooth, for what? Asking questions? These angels are really out here physically assaulting other angels, murdering them for having the audacity to care about God's creations, and he's the one who was cast out of Heaven?
Aziraphale's time down in Hell all seems rather fun, like when you get out from beneath your controlling parents and get to pierce your nose and dye your hair and get a cat to call by funny names. He's playing the cool, handsome and suave demon he thinks his ineffable husband is. He made Michael miracle him a bath towel, hah! But he doesn't have the same strings attached; well of course everything's a bit shit in Hell, and the demons are, well, demons. There's no surprise there.
And sure, Crowley knows that Aziraphale's boss sucks, but he's still an angel. His lot writes rude notes to get the point across. Angels aren't meant to kill other angels. It's a bit like watching your sibling get crowned The Golden Child Who Can Do No Wrong, then turn around and pawn his mom's rings on her birthday for cash, while the worst thing you've done is get snarky from an overabundance of teenage hormones (not that I have, er, personal experience or anything). And she still favors your sibling over you.
Believe me, that sort of thing cuts exceptionally deep. Like, really, you chose them over me? Why them, why not me? What have I done? What's wrong with me? How is this considered acceptable behavior? How is what I've done so bad, compared to this? Are you kidding me right now?
All for having the audacity to ask questions and imagine things beyond the guidelines he was given. The same reason humans were cast out of the garden, sent into the barren wasteland, where they imagined things beyond the hell they had been banished to and raised villages and cities and an entire civilization that culminated into ripping Satan's own child from the hands of hell and molding him into human incarnate.
