[RedMattis]

Mages and using magic in the setting is (technically) outlawed. Most of the powerful mages are ageless and either bored or entrepreneurial. Even before this mages were well known to create various chimeras or worse (undead f.ex.). One rather unimportant mage the players came across in a mage-refuge created human-spider chimera servants for himself to manage his magical spiders that produced silk which he sold for use in expensive magical clothing. The same guy was looking to create human-spider chimeras which could produce the same silk, but at least that mage was smart enough to kill or keep the "failed" babies instead of just letting them out in the wilds like some other mages.

My main empire might be a bunch of xenophobic religious fanatics, but their reasons for deciding to ban the mages guild and execute one of the guild leaders were fairly well justified. Didn't help things afterwards when said guild leader knew and spoke some kind of Word of Power that killed himself most of the royal family and about a thousand innocent people.

A Wizard Did It is pretty much the standard explanation for strange stuff. Gods and "Demons" are even worse though, and are often at fault for the real disasters.