Junior brother's mother, Song Jiao, had willingly become a demon cultivator. Her insanity, in this case, was to kill.
No one knows why she had decided to fall.
She was slayed by a cultivator.
Fang Zhou had repressed his memories, but as he continued his cultivation, bits and pieces slowly came to him.
He blamed demon cultivators for his mother's fall.
So, he decided to kill them all.