He is Slytherin.
He is Cancer.
He lives behind a towering wall of prejudice, sarcasm, cold, and hatred - it is his wall to put distance between himself and hurt, to match the frozen emotionless walls of his family, and to keep his brother out, and his own feelings in.
Regulus reacts with frosty derision and clipped phrases. This wall is composed of the words of his parents, his peers, his housemates, the ideals dripping from the lips of Death Eaters he admires.
He has taken their words, their opinions, their purist upbringing, and formed it into a shield for himself.
Behind it, he quivers and quakes. Cancer, a crab, a harsh shell guarding a soul inside that is so weak, vulnerable, needy, and so desperate for love and respect.
Cancer.
A Slytherin cancer gnawing away at his soul, corrupting his mind until he cannot even hear his own voice, engendering hatred for the brother he loved - and rotting away his legs, even as they scream at him to stand, be proud, be strong, never shame us!
His mother's voice, dripping poison into his veins. His own voice beating himself down as he tries to find a place to stand.
There is no war, no cause, no purist hatred of mudbloods.
There is a shell, and a boy lost behind it, orphaned of the vibrant brother he loved, left in darkness and the colour Black and a guilty stain of which he can never be free.