Ryuwhan hates witches on principle because his master was killed by a witch. We are led to believe that he refers to Edea Florence, and that the witch responsible is thus Aria, but it later becomes apparent that his master is someone else: namely, it's the Great Witch of the West. Knowing this, his stated motive for his MO is much less reasonable considering that West was the most powerful witch at the time, and it took two other Great Witches working together to take her down, and one of them ended up depowered immensely. Knowing how powerful his master is, an astute individual like Ryuwhan should have realized immediately that there is a very small list of possible culprits who could actually have killed his master.
The Halloweens. Initially appeared as Man-made supporters who serve their masters. However, the horror kicks in, when the readers find out these pumpkin-head dolls used to be human. Fergus, Ryuwhan's Halloween, is implied to be a criminal, and this was his punishment by West. However, what about Mordred, who's shown nothing but be a kind-hearted girl, weighed down by responsibility? Unlike Fergus, her memories were sealed and is under a spell to love Tasha unconditionally. Plus, who knows how long she's been turned into that? Judging by the decline of Britain since her absence, it's been decades at least.
