I am also pretty blind to most things outside of the... non mainline SW movies. And some of the games.

I'm sure Heero had a normal life once... but it was gone by the time he was like 4 or 5. After that he was raised by an assassin type who definitely started training Heero in the family business from entirely too young of an age.

But he's got a good heart under all militantness I swear! Even if they did try very hard to program the humanity out of him. They didn't actually succeed. They just made him awkward AF.