I don’t really buy the whole “Gabriel Reyes sold out and destroyed Overwatch out of jealousy” meta because that’s just. Such a lame and overused story trope.
What I do believe is that Gabriel planted himself right in the middle of Talon and all their trouble in order to act as a spy. Sure, when Jack accepted his rightful role as Commander of Overwatch, it must have greatly hurt him- but this is Gabriel Reyes, the man who led the strike team responsible for saving the damn world against the omnics.
It doesn’t fit that he would throw all of his efforts, all the work and blood he spilt to save the entire world out of the window just because of a title and position.
Moira makes this all the more complicated, considering he invited her into Blackwatch- but what if he was doing it to keep an eye on her?
Keep your friends close, but your enemies closer. It’s the oldest trick in the book. And in this image:
He’s utterly horrified. My theory is that this was a sort of initiation for Gabriel to join Talon- offer his body for Moira’s experiments and that would prove how loyal he was.
There’s a lot of fishy stuff going on, and in my opinion, no one can really conclude that Gabriel is utterly evil- not until more lore is given to us.