Contrary to the beliefs of many, she was not stupid. The Black family was known for madness, but it hadn't gotten her. Her son was taken early. She had her first inkling that something was wrong when he was only nine years old. He wandered off while she was arguing with an incompetent seamstress and started explaining the Hogwarts house system to some Muggle girl getting her school robes.
It shocked her to silence and they hurried away from the store without finishing the order.
Now she knew it was the first sign of madness. If only she had known, perhaps something could have been done and she would still have her darling boy. She rolled her wand in her palm - an old nervous tick.
Perhaps they had missed that sign, but when he was sorted to Gryffindor there was no question that something was wrong. But they had ignored it again. She had decided it was the rebellion of youth and was angered by the danger he unknowingly chose. When he grew out of the rebellion he would be in a dangerous dormitory and spending time with dangerous people. He would need to use all the Slytherin cunning he had deeply hidden to survive. She yelled and screamed and railed at the stupid boy that put himself in that position. He never grew up. He would never grow up.
When her darling Regulus started at Hogwarts she began to accept the madness for what it was. Sirius loved his brother through the teenaged rebellion. But now, there was true animosity. She began to think that Sirius truly believed the things he said. She tried to teach him the truth. She tried to make him see the disgusting reality of Muggles. They were dirty and dangerous and they stole sons from their mothers. He would not break from his demented ideas.
She rolled the wand back into her wand-hand and placed the tip delicately on the face of her lost son. The son that slipped through her fingers before she even knew she was trying to hold on.
"Goodbye my dearest boy." Flames erupted from her wand and scorched the family tree in a dark circle where he son used to be. The Potter's could have him now and feed his delusions.