I do like @HubCityQuestion’s idea of starting in the past, and what his life was like before he was chosen. Backstory-wise a lot would depend on which version of the character you are getting. He’s had at least three different origin stories even before the movie.
The earliest was simply a matter of power eventually corrupting him and making him go bad
The second in JSA he was shown as having been a hero for many years alongside the first incarnations of Hawkman and Fate. He apparently went off the deep end after Hawkman and Hawkgirl were murdered. He took back Kahndaq and ruled for some time before the wizard imprisoned him claiming that he was becoming a tyrant, when the gods refused to let him strip Teth-Adam of their gifts. Apparently the gods themselves believed his actions were just. (I have noticed the wizard can be a bit of a tyrant himself…and he doesn’t take kindly to disagreed with)
The slave origin seems to be the most recent of the three, and harkens back somewhat to the power corrupts type of storyline. He did what was necessary but took things too far, and was turning into the very thing he once despised.
I must admit I am most fond of the second incarnation myself, and there’s a lot I could do with that backstory-wise. I think the most recent incarnation of the character stripped away a lot of the things that made him a more sympathetic antihero style character and tried to pigeon hole him into being a straight-out bad egg. If you paid any real attention to the JSA to 52 version of the character, you start to understand just why the poor guy went off the deep end. His entire family was murdered not just once but twice. That version of Black Adam had good intentions at heart when people let him be and he wasn’t pushed too far or backed into a corner.