Actually Atomic Guy was an old scientist that exposed himself to radiation, which eventually turned him into astro guy. he eventually died from radiation poisoning and his succesor (Atom Guy) was a wrestler that fought against Famardy in the second game. They both want to save the earth, but their intelligence began to waste away, until they eventually became monstrous and savage. They don't fight for fun and they end up causing more harm than good.
Uhhh...something about him being a nuclear physicist and... nuclear accident... blah blah, BUT whilst giving him super size and strength and shit, it's also slowly melting his brain... so I think he just fights to have fun for the rest of his short life.