Quick note, I just want to say that I am in no way trying to justify all the horrible things Adam did to Blake and Yang throughout the series. The writers clearly made him be irredeemable in what's he's done, though I don't agree with everything that was done with his character I can respect the writers want not to redeem him. However, I am very intrigued by what was revealed to be under his mask, so I wanted to write this. Enjoy.
He'd grown tired here. Tired of working for one of the Schnee Dust Companies many labor forces. Mining the dust here at the quarry for it to be refined and shipped by the railway. It was hard work, tiring and less than rewarding for him and everyone else here. When Adam looked around the dismal setup and among the workers, all he saw were the haggard and worn faces of Faunus trapped under the questionable and strenuous working conditions. Faunus, beings no different than what he was; people no different than anyone else. Yes, that's what they were: people, not animals. People no different than Jacques Schnee, or anyone working for the Schnee Dust Company for that matter. Yet, why were they here doing the dirty and dangerous work? Treated unkindly and pushed so hard? Why, when they were just people trying to live their lives, were they still after all this time still weighed down by the human's oppressive actions?
Adam made those observations often, until one day, he decided that he and any other Faunus here deserved better. On that day he stood up, refusing to go on with the work that only served to weigh and beat him down so often. He spoke about it too, encouraging his fellow Faunus to take a stand with him, to stand up and refuse this maltreatment. There had to be something better in the world than this, humans and Faunus had to be able to coexist in peace. They were not subservient, not submissive, and they were not subhuman. They were people, living and breathing just like everyone else. They were equal.
But maybe he'd been too imaginative. Too hopeful, after all, what could a kid like him really do? Maybe that was why things didn't go well, the Faunus were only punished for their rebellion of sorts. And Adam, the appointed ringleader, wasn't simply let off the hook and loaded with more work like the others. No, he could remember grabbed and dragged, no room to break away no matter how hard he fought.
"Maybe this way you'll remember who you belong to."
Eerie voices of evil men trapped in his mind. He knew then that the dust company, the Schnees, man, in general, did not believe in this idea.
"I wouldn't be doing this if you'd just behave."
And he heard their words as they held him like the animal they truly believed he was and pressed that hot metal into his face. Laying there, pinned down as a sourcing brand was scorned onto his face and over his eye. As struggling grew fruitless he was forced to lay there, his eyes forced to focus on the brand of burning metal was lowered so painfully slow to his face; forced to lay there and take the unimaginable pain that he had failed to ever feel before that moment. They left the initials of Schnee Dust Company burned and forever permanent, trapping him to carry the letters of the company he hated so much.
He knew what they did to him had supposed to be a lesson of obedience, but all he learned that day was that the oppression, hatred, bias, and sub treatment towards Faunus was real, deep, and true in the hearts of humans. And when he left, got away, and found the White Fang, he did not forget. For the rest of his life, he had unwillingly been given a reminder of the human's ugliness. He was forced to relive the anguish and fear and every mirror only showed him that anguish and fear through the form of that branded scar. So he did the only thing he could and hid it all, under that mask. The light of hope for them was gone, he could pretend, but pretending only got so far when such a burning hatred sat inside him. It came at the expense of those around him, of which he only continued to do harm to.
The fixation on his past only ruined him and those around him. It was the source of many people's pain, as well as the thing that ruined and led him to death in the end.