She didn't know

[RWBYFangirlForever]

Misery was blind from birth. Her mother had died recently and she never knew her father. All the other children picked on her so...
She turned to black magic.

She learned from an old witch in the woods. A demon witch.

Slowly, the more Misery became addicted to this black magic, the less soul she had. Until...she became a murderous villain. The demons would whisper and tell her what to do. They were her eyes. She did whatever they told her to do. She would kill for her ability to see and be normal. So that's exactly what she did.