THe TL;DR: The UN are the bad guys, Blackwatch was based on Talon, Overwatch was a propaganda tool and the Swiss HQ explosion was assassination.
At the fair founding of Overwatch and Blackwatch where we lay our scene. Gabriel, despite being more qualified to leading the organization, is bumped into the sister-organization Blackwatch to operate underground.
The UN does this for two reasons: 1. The real heavy lifting was always designed to be carried by Blackwatch, and Gabriel is more qualified and 2. Overwatch becomes a face for the UN, and while Jack is a capable commander, he’s mostly there for show. Their golden boy.
This is not to say that Overwatch isn’t actually doing work and genuinely risking life and limb. But they are shipped out to the higher profile areas, literally leading children out by the hand on tv. Overwatch had one primary function. ‘You are safe; We are handling this’.
I think that Talon (or it’s early form) had infiltrated the UN. Blackwatch was doing the grunt work, but was increasingly turned to more serving the sovereign wants of the UN and/or Talon. They make sure that Blackwatch is kept under control, and have an agent that works for them within their midst. Jack is kept in the dark about this and Gabriel is under orders to remain silent.
This of course creates a schism between the two. Jack thinks they’re there to save the world and be the heroes and Gabriel thinks Jack is too naive and idealistic, and Reyes is angry that he can’t see that they’re being used.
Then Blackwatch gets exposed. The public goes in an uproar. But no worries! The UN is gonna launch a super good sekrit investigation, ya’ll. There’s nothing suspicious about investigating your own shit that’s circling the drain, is there? I imagine at this point, that Jack demands information, which Gabriel cannot give to him freely.
McCree leaves Blackwatch right before everything goes up in flames (literally). Gabriel, the man with the most intel, the person who can effectively sink a whole bunch of higher ups, is successfully assassinated.
Instead of emerging as a victorious hero like he has so many times before, Jack fakes his own death. Which makes no sense unless he was never meant to walk away from that explosion and the hit was meant for him too. He goes out on his own, undercover, investigating something as the jaded Soldier 76.
Mercy gets ahold of Gabriel’s body and resurrects him, but he’s captured by Talon because of his strategic importance.
McCree meanwhile is on the run, with a bounty that could probably save the economy of a small country and OH BTW. BEING OVERWATCH IS NOW ILLEGAL. Any of you assholes get back together, we are gonna lock you up.
Everybody that could have information about this is effectively driven underground and outlawed. Aint that a peach.
But hey, the UN said that the explosion was caused by infighting. That is, after they amended their story from ‘there was no foul play’.