Could Dmon Become a Villain?
Looking at the lore, I don't think the possibility can be completely ruled out.
Dmon and Dva were once on the same StarCraft Pro team. After retiring from esports, they enlisted in the military together and eventually served in the same MEKA Squad. She even chose a callsign similar to Dva's = Dmon.
Based on the official lore alone, D.mon has already spent most of her life following Dva almost obsessively. It's even possible that the two grew up in the same hometown and same school.
The problem is that Dmon was always second best.
The spotlight always belonged to Dva.
For most of her life, Dmon lived as little more than Dva's shadow.
Dva sees Dmon as her dependable childhood friend and trusted comrade in arms. But beneath the surface, Dmon slowly nurtures a twisted obsession. She silently promises herself that one day she'll surpass Dva.
Then fate presents them with a decisive crossroads.
Inspired by the MEKA Squad's success in resolving the GWISHIN crisis, Overwatch decides to recruit a new member to join its ranks.
Dva is selected as an Overwatch agent.
Dmon is not.
For the first time, the world officially declares that Dva is a hero...
...and Dmon isn't.
Unable to accept that reality, Dmon eventually joins Talon, Overwatch's greatest enemy.
Ironically, within Talon she finally earns the recognition she could never achieve while serving with the MEKA Squad. She is acknowledged as one of the finest pilots alive.
Yet none of it brings her any satisfaction.
Because Dva isn't there.
Eventually, Dmon reaches a terrifying conclusion
Dva stole her life.
Now she has only one goal.
To destroy Dva and the incompetent bureaucracy called Overwatch that chose Dva over her and prove to the world that it made the wrong choice.
(Friendly Encounter Dialogue 1)
Dva: Yuna, you again? Why do you keep showing up wherever I go?
Dmon: I go wherever I want, whenever I want. Try and stop me.
Dva: At the very least, stop messing with the things in my room.
Dmon: I-I never stole your bunny plush!
Dva: Stay away from me, you creep.
(Friendly Encounter Dialogue 2)
Dva: Yuna, it's not too late. You can still come back to your friends.
Dmon: Friends? You mean your minions?
Dva: Yuna, you've misunderstood everything.
Dmon: It's too late. Even if I wanted to go back, I couldn't anymore. And besides... I'm never going back!