should have shown..

Yeah coulda been cool. Wonder if they had tried to film it and it sucked or if they thought it would just be better off screen

I don't think so, it keeps things ambiguous and would screw up the suspense because at that point Reagan has not been shown as full demon.
The scene obviously would have been interesting. A sick Reagan calling out for help and Burke goes up to help. The door is locked behind him without him noticing and then he's tortured with psychological games by the demon until he's broken down and crying. Who knows if the demon even needed to throw him out the window or if it persuaded him to do it himself. Of coarse the head turning around would have been hard to explain without direct contact.

I prefer the ambiguity and mystery of just hearing about it. We’re still wondering how bad Reagan’s ‘condition’ is. We’re like ‘did she really murder someone..!?’

Then as things go from bad to worse, and we hear about the 180° head twist, and then see Reagan do the same, the horrible truth reveals itself - ‘holy shit, she did’