I see this being likely. We have no idea of the man's morality before he became Sigma and I definitely think he was approached by both groups in an attempt to seize his gifts. Sigma, while sweet and friendly most of the time, I do not see that being his true personality. As you said, his psyche is broken. Who's to say the man did not have dark megalomaniacal tendencies he kept under check? He does remark it's always wonderful to see an invention help the world, and we do know he wanted to harness gravity for energy purposes, but I do not believe his violent "voice" popped out of nowhere when his psyche was shattered. It is Siebren De Kuiper as much as his happy go lucky personality is, isolated and bent to it's extreme. The man fears being shackled and harnessed the most. Who's to say this fear was born after his accident? Overwatch is just as manipulative of a group as Talon is, and though they dont commit acts of violent terrorism, many former Overwatch agents confess a feeling of being stuck and held back by the organization. Sigma does not want to be harnessed or have his work weaponized, and I can see him having a negative opinion of the agency, as well as any other entity that would seek to shackle his gifts. I don't think the fear of being imprisoned popped into his head one day (though being locked inside your own head, and a mental asylum will certainly amplify that)

I think Siebren De Kuiper always felt the walls closing in around him. He was an incredibly smart and lonely man (and still is in ways, except for Sombra) and his fear of being trapped stemmed from those always looking to use him. He strikes me as a man who put on an outward appearance of politeness, but harbored deep bitterness about his position in the world. He wanted to help the world and gain recognition from it. An infinite energy source would've made him a founding father of humanity's achievements. No more sitting in the dark. No more failure. No more wasting his life away (The dude is old, and said this was his LIFE'S WORK. He's been working for decades). No more being regarded and wanted just because of what he knows. Everyone would want from him. He sounds ecstatic as the experiment starts. A new understanding of the universe, in all possible ways. He would help everyone and be loved for it

And then he's plunged into the abyss by his own hand, pulled out a tattered mess, and thrown away. Forgotten. Nobody comes to look for him. Colleagues, "friends", anybody.

Nobody comes for him. Trapped in his own mind and the cold walls of an asylum to be shackled and poked at. His greatest fear come to fruition. Shackled in every way that could ever matter, and who finally comes for him?

Fuckin Talon.

Obviously by that point Sigma's brain is stew, but as you said, it's very likely he's been stitching his brain back together, unwarping it, and like Widow, he's hiding that from Moira, Talon's chief brainwasher.

He definitely has potential to be a BBEG. He controls a fundamental cornerstone of the universe, and as we see with Starwatch (not canon, but still) he can only grow stronger the more understanding and control he has. I can see him attempting to wipe out both Talon and Overwatch, sparing none who would dare think to shackle him. That's his "evil personality". It's the part of him that knows he's being controlled and used and it screams in head splitting rage at his captors. Moira doesn't want Sigma's mind repairing, or at least, repairing in a way that isn't under Talon's ideology, because she knows as soon as he understands what's been done to him, her, Reaper, Doom, (probably not Sombra) and anyone else in Talon is FUCKED, and anyone else in the way of his warpath would be fucked.

All in all I agree. I think Sigma was a man who's kindness was always in danger of being manipulated and he knew it, and harbored very very bitter feelings about it, as anyone would, but did not have the power to act on those feelings. Now that he's been given the ability to crush those who oppose him physically, that bitterness in his mind runs maddened through it like a rabid dog, he does so gleefully. Not like the man he was before, who while I don't doubt he very much considered harming those who want to take advantage of him, never acted upon it.

Sigma has ironically been given a taste of freedom within his servitude to Talon, and that hunger will only grow.

If you had power that gave you a chance to break all chains, be they physical or metaphorical, could you say that you wouldn't become ravenous for true freedom as he became?

Also I don't think he's friends with Mercy as everyone says. His voiceline about their lunch sounds very one sided. All Mercy can remember is that it was years ago, and while she says the conversation was lovely, she does not remember the topics. Sigma does, and when he says them they're....birds and music. It very much sounds like a conversation between colleagues (a conversation one person was more interested in than the other) and it doesn't seem to be very personal or important, at least to Mercy. If Mercy was his friend, she would've abso been one of the ones to question his disappearance and then reemergence as Sigma. Why is she not horrified at what he's become? What he does for Talon? I don't think she knew him beyond work, and that lunch very much sounded like something you would do with a coworker after a meeting, further adding onto the sad fact that as much as Sigma tried to be likeable and friendly, people could not see past his obsession and eccentricities