He broke the chain. Our family's proud chain of Slytherins was ruined when he got sorted into Gryffindor. Regulus felt betrayed by the brother he had looked up to. His brother had always been like a hero, he was good looking, he was cheeky, he always knew what to say and do. Regulus had always tried to be like him, he had always succeeded. He was a copy of his brother, but now, now when that brother had betrayed them all, he couldn't look himself in the eyes anymore. How could he ever have looked up to him?

Regulus felt betrayed

Regulus heard loud and clear as the hat yelled "Slytherin!" and as he came home for Christmas, happy and proud for his house, for his family, for living up to it, his mother met him with a smile. He had never had as many presents as he had that year. His mother had never noticed him that much. He was no longer a copy. He was no longer the shadow. He had broken out from his habit, he was now over shining his brother.

Regulus felt loved

The mark hurt. It itched, it ached, it showed. Clearly. He didn't like it. He hadn't wanted it. Sure, he agreed with his parents. The world was filled with mudbloods, but did he want to kill them? No.
That was the first time Regulus broke from the line. He wasn't like them, but he didn't notice. He had gotten the mark, he was a Black after all.

He sat on his bed, he held back tears. A black didn't cry. A death eater didn't cry. He didn't know what he hated to be the most anymore.
Sirius came in and sat besides him, he placed an arm around him and hugged him tightly until Regulus couldn't feel the tears anymore. He loved his brother. He loved the brother who had broken the chain, hated everything about it, and still supported him even though his was just like them.
How could he have hated his brother? How could he have tried to break him, for breaking out?

Regulus felt horrible

Regulus sat on the floor, staring at the burned circle where Sirius face used to be. A face with this cheeky grin and glinting eyes. He had finally done it. He had pushed his brother away. The forgiving Gryffindor had turned against him.
Regulus couldn't help but laugh, a cold laugh which turned into a sob as the tears stained his face.
Sirius had always been the Slytherin, he was cunning, he was sly, he was proud. Regulus saw that now, and maybe that was why the Gryffindor had pushed him away.
What made him cry even more, was that he found out that he was maybe more of a Gryffindor. He was loyal. To his family? No. He betrayed them, but they weren't worthy of his loyalty, the bloody mark and everyone who bore it wasn't worthy. No. He was loyal to his brother.
And he was brave. Even he could see that. What he was going to do, defy the Dark Lord, it was stupid, but brave. Just like Sirius would have done.
But he had failed him, he had disappointed him just one too many times, and now his brother was gone.
He wondered if that was why he did it in the end, why he had the courage.
"I do this for you" he whispered as his fingers trailed the burned circle.

Regulus felt lost

Regulus was in pain. He could feel the ache in every muscle, every bone, every nerve, in his body, but that wasn't what pained him.
He could hear his mother yell at him for betraying the family, breaking the chain. He could see her burn off his face from the family tree, but that wasn't what pained him.
He could feel his lungs scream for air as he as dragged under water, feel his legs and arms fall out of joints as something pulled in him from every direction, but that wasn't what pained him.
It was Sirius face. Sirius who was disappointed once again.
He had failed, he hadn't destroyed the horcrux. And now he was dying.

Because of him.. What he had done, what he had made him realise
Regulus felt broken