Douman Ashiya was truly an evil person.
He was actually far more evil than in the fictionalized stories about him. He was unmistakably and incurably evil!
But, that would only be from the normal "human" standpoint.
Douman Ashiya was incredibly cruel towards humans, including even his own children. He basically didn't have a single moral value that normal people would have, which was why he could only be described using adjectives such as "insane" and "evil". However, he was actually quite gentle and magnanimous towards non-humans—basically, demons and other spiritual creatures.
Normally speaking, if there was a fight between humans and demons, spiritual ability users would be on the humans' side. However, Douman Ashiya was the opposite. He always acted for the sake of demons. In fact, there were even multiple confirmed examples in history of him slaughtering an entire village for the sake a demon.