Why did he pull down his zip? *spoilers*

I wondered about that too, i came too the conclusion it was so Roger would be compliant!
Brennan asked him for a blow job, then when Roger went down facing him, he told him to turn around and while Roger was turning around thats when Brennan unzipped himself!
Roger then relaxed allowing Brennan to get to his neck easier to strangle him.

There were some pretty strong hints that Brennan was gay, and this was one of them.