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I preface this by saying I haven’t read most of the Horus Heresy books because feth you there’s like a thousand of them. But I have read Fulgrim. Among other things it shows the gradual transition of the Emperor's Children to full time Slaanesh worshippers, such that by the time of the drop site massacre they’re full on noise marines and wearing people’s faces and such.
Now I had always had the impression that being turned to chaos was a sort of slow and incremental process, but now I’m wondering if that’s true. Like I said, the corruption of the Emperor’s Children seemed to happen slowly. So what I’m wondering is how the hell did this play with the legions who had just joined Horus for the drop site massacre? I’m imagining one of the traitor legion members going up to another and being all “Welcome to the rebellion brother. You’ve taken a big step in swearing allegiance to the Warmaster. It’s going to be hard to fight against our brothers who have yet to see through the lies of the Emperor, but we must have the strength to do what must be done to save the galaxy. By the way, we’re clothing ourselves in our victims faces now. Pass it on.”

I think part of it was simply the inherent contempt the traitor legions felt for each other. Horus could hold them together, but there wasn't any camaraderie.

So the response to seeing the EC in their depraved state would be an eyeroll and a "yeah they would do that kind of stuff"

That was one of the reasons I disliked the early Heresy books after Horus Rising. The speed of the corruption was too fast, and things were too obvious. I think it is ultimately down to poor and inconsistent writing.

By Flight of the Eistenstein you already have full blown Plague Marines. I thought that was way too much too fast.

Angel Exterminatus covers elements of this as Perturabo is shocked by the behaviour of the Emperor's Children. By the end of the book, he witnesses Fulgrim go full on Daemon Prince. While this is also shocking, he sees the power that Fulgrim gains in the process.

I think there is an element of deciding not to be left behind. If he is going to turn traitor, he may as well commit 100% and get the rewards rather than hanging back and being left in the shadows.

Running willingly full-tilt into corruption's waiting, eager arms when all 4 chaos gods are snatching up new followers is the fast track. And you don't see it really beyond the Horus Heresy

The slow roll corruption i'd say is much much more common.