Main Canon (Space/Amun)

I don't know about his childhood, but by his mid-teens, he joined a large spaceship crew—basically an entire city in its own right. He joined it due to there being less regulations in space, similar to the oceans of Earth, where there aren't true governing bodies. The ship mainly did mercenary work, traveling the galaxy, and Deimos quite enjoyed it, both for the adventure and freedom.

He went to school on the ship, while already interning on various missions. He was a scientist, first and foremost, but tended to get his hands dirty, traveling to various planets and looking into all sorts of phenomena, both for business and pleasure. He'd also build plenty of giant machines to do various (usually destructive) tasks on these planets. One can liken his view of these planets as Africa colonization on Earth: your average heartless eccentric psychopath wanting the freedom to do immoral things in a "lesser" civilization, due to a large gap in power between his superior knowledge/technology and theirs.

He enjoyed this life, but considered this ship a stepping stone for a future where he was fully in control, not even bound by the meager laws the Coalition had.

Meeting Amun

At some point—probably within a few years—he meets Amun, another student, sent on the ship by his disappointed family, in hopes a life more on his own would "fix" him. Deimos, being his usual talkative self, is the first to strike up a conversation with him, and given Amun is too shy to ever talk to anyone else, Deimos eventually becomes his only friend. In time, Deimos learns about Amun's depression and self-esteem issues, which he ends up liking, since he can use it to his advantage, making him his guinea pig. Amun accepts, glad to be of use to someone for once. An unhealthy close bond forms between the two, and they end up falling in love for each other, both for bad reasons: Amun is too awestruck by Deimos' "kindness" (the fact that he's always talking to him) and too depressed to realize he's just a tool, while Deimos loves Amum as a project, not a person. In fact, Deimos finds Amun's personality his only flaw, and isn't shy about telling him such. Amun agrees.

With Amun as the perfect willing test subject, Deimos dedicates a lot of time to "improving" him, enhancing his body with wings and enhanced muscle growth, turning him into the supersoldier he is today. He's confident that with Amun's body and his mind, almost nothing could stop them.

He didn't plan on running from the Coalition yet—wanting more knowledge and power before making the leap—but when a conflict arises with another planet, he is confident enough to offer a plan of breaking into the planet's moon, taking control of the god that resided there, and destroying the planet with the moon. Others on the ship found his plan too risky, but he decides to go ahead with the plan anyway, wanting to prove himself capable.

Him and Amun succeed, but with an entire planet destroyed by their hands, the whole solar system is suddenly their enemy, and the Coalition will do nothing to defend them, as they were against their plan from the start. Thus, Amun and Deimos are forced to flee and go into hiding, Deimos' plans of true freedom starting earlier than he had hoped.

Becoming Amun

Regrettably, the two slow each other down in various ways: Deimos is a liability thanks to his limited supplies to create new tech to defend himself, and Amun still has his depression and aversion to violence getting in the way of making sure they eliminate everyone who's a threat. As such, Deimos decides to do something he had thought would be useful for a while now: he returns to their old ship (sneaking in) and inserts his own mind and soul into Amun's, becoming one with him. Combined, they are the perfect being, according to him: the sheer power of his ultimate creation, while having the greatest mind to ever live: himself~.

"They" fly off again, and start a new life far from the many governments after their heads.

From here, Deimos doesn't quite consider himself Amun, but he never outwardly states Amun and his "condition" to others, so most consider Amun is just Amun, finding his occasional referencing of himself in the third person just an odd quirk. With no true "enemies" made yet with this new population, he's a lot friendlier, and spends his time exploring various planets, doing whatever he wants. Often he'll even find himself helping people, because he enjoys using his new body's abilities to their full extent. Most consider him a pretty nice guy, outside of the fact that he won't hesitate to kill you if someone gives him a sweet enough offer.

Outside of that, for the time being, he's a lot more neutral-to-benevolent, Deimos no longer having a crew to fall back on to be the truly malevolent man he'd like to be. He also just wants a break from all the running away, and enjoys the simpler life. But in time, he'll surely be back to his "evil supervillain" dreams, once he finds a new entity he can utilize as another stepping stone in his quest for ultimate control and freedom.