I was going to save this for my fic but whatever....
Moira didn't ""create"" Reaper. At least, not completely.
Micheal Chu confirmed a long time ago that Reaper as the world knows him has been around for decades. But that would mean that Moira would have had down experiments on him and what not long before the end of Overwatch was in sight. So I think there is something more to the Reaper origin story than one person messing around with his genes.
I think SEP is the answer. We know the experiments were done to make the perfect super solider, so wouldn’t it make sense if one of them made Gabriel effectively immortal? In canon, Reaper has regenerative abilities, which would be far more desirable in a soldier than intangibility. I think that the actual results of the experiments wouldn't be too major, but they would be enough to make a noticeable difference. Just enough that there would be rumors of this guy showing up at terrorist sites (you know, for Blackwatch stuff) and acting less than human.
Then Moira comes along. I think Gabe hired her to weaponize Angela's research. My first attempt at writing this post was ruined when the app crashed, but at least this time around I don’t have to speculate about Moira using Mercy’s research. Micheal did that for me. What I want to point out is that Gabe looked terrified in her backstory and the signature smokiness of Reaper is really what seems to do it. I think Moira attempted to improve on what abilities Gabe already had and to disastrous results.
But I don’t think it was an accident. Because Gabe looks scared, but Moira looks proud.
She definitely intended to do it to him, and I think it has everything to do with Talon infiltrating Overwatch. Talon had to take out as many of its leaders as it could before bringing the whole place down. But while they could kill Gerard, they could be fools to get rid of Gabe, especially when everything about his method of action coincides with what they stand for. I think agents like Moira wanted to isolate him from his allies in Overwatch and make him feel as though the people who ended the Omnic Crisis with him could no longer be trusted. I think “butchering” his humanity was intended to bring him one step closer towards alienating him from the rest, to start forcing him to view everyone as potential enemies.
Because how do you tell the Strike Commander and your closest friend that you can turn into smoke? How do you tell them you wanted it to happen?
And yeah, that’s it. I’m glad I could stand here and spoil RR for you :)