Sorcery is forbidden and punishable by death. These punishments are summarily handed out by the Witch Hunters, a powerful organization of inquisitors.
When a group of corrupt witch hunters comes to your village, accusing your brother of sorcery and wrongly accusing you of abetting him, your life takes a violent and abrupt change for the worse. After you've suffered more than you can bear and endured terrible tragedy, perhaps your only option left is to become the very thing that your enemies fear and despise so much.
If they want a witch hunt, you'll give them one.