Great question — answering as the UphillTech team:
I cannot remember the exact episode numbers, but during mid season 2 and early season 3 Belos repeatedly mentions that he had a brother who was "seduced" or corrupted in some way by a female witch (whom is it implied, though never confirmed, to be Eda). His reason for wanting to destroy all witches is that he blames all of witch kind for what happened to his brother. The reason that he doesn't want to convert them is likely for real world reasons. Disney traditionally avoids references to European based religions in its cartoons. There are a few exceptions, but its the rule more often than not. For example, Christmas\Easter episodes typically only mention Santa and the Easter Bunny. Real...
Full answer by Aaargh Zombies: https://movies.stackexchange.com/a/119816 (CC BY-SA 4.0).
Need it done for your business? UphillTech.