I personally took it as her reverting to full blown Ice Queen. She was coming out of her shell a little with the MC and this jolted her back to what she was at the start of the game. Cold, distant... pragmatic.
Obviously that is just my take. The game doesn't go out of its way to prove she's reverted (she's the same afterwards regardless of that betrayal) so it just as easily could be lazy writing but if we're being generous it could have been deliberate.
That scene was probably DPC trying to be like George R. R. Martin once again, not realizing that pulling the same stuff over and over again is getting stale and predictable. I knew something weird was coming as soon as Cathy took off her panties before the blowjob (the logic is so hard to find in that scene, that even the gods of theorycrafting
Warscared and
dalli_x didn't even comment it).
The difference is that George R. R. Martin's shocking scenes are hinted in foresight, like if you look into the details of the story, you might see it ahead of time. DPC's shocking stuff (especially in AL), are super out-of-nowhere, like he just does it even if it doesn't make sense and he's going to be like, oh I wanted to have that emotional effect (and get offended when people diss him for the execution of the swerve). Like he has to realize not everything in the story has to be a swerve (what is this supposed to be? Late 1999 WCW?) and some stories are better told straight up.
Like Shakespeare wrote a lot of shocking stuff and he didn't pull a swerve every chapter, and what do you know - the guy ends up being the most well-known author in history. Plus he was kind of obsessed with killing off characters (for example, like only 2 people survive in Hamlet).
P.S: Do you know what else has like
a lot of death in it? The Bible. Like EVERYONE dies in that thing. Like Death dies in the Bible. Idk, if DPC would go that far, but I wouldn't be surprised if he tries to outdo the Bible.