Conflicting Gabriel Reyes theories:

– double agent or his involvement in talon at least started out this way. Lots of convincing people he’s ‘seen the light’ of talons way of doing things/he’s sick of overwatch/not getting credit/etc but none of that’s actually true. Or he does this under the identity of reaper and talon doesn’t know that’s also gabriel. Possibly trying to help Amelie lacroix as well. He’s trying to discover who the moles within overwatch/blackwatch are and get them out of the picture before they can do too much damage. Obviously this doesn’t work but he’s still doing exactly the same thing and still on the hunt for overwatch people who helped talon back then.

– Gabriel’s job has always included going into places and making things worse so overwatch are asked to or have an excuse to intervene – he does this as reaper (who’s been seen on battlefield for “20 years”). Again he’s still doing the same thing, he realises talon needs to be a big enough threat that the world will accept the reformation of overwatch, his attack on watchpount Gibraltar forces Winston’s hand on the recall, etc.

A healthy side of ‘becoming the mask’ in either case undercover work is hard (and he’s undercover in both senarios and has basically been doing it alone for six years) but also possibly via: to borrow from gameplay, Moira’s experiments mean killing people aliviates pain/makes him feel good in some way. Which could be the inspiration for widowmaker. Or he’s convinced talon/Moira sepcifically that this drive exists or is larger than it is to alleviate suspicion.