The demons are so incompetent, that a normal human can stomp them lol. The twist with the exorcist knowing what Genma is was so unexpected, seinen manga always go in interesting directions.
I'm guessing it's because he knew she's a second-generation demon. We were told in previous chapters that second-generation demons aren't governed by the same rules/laws as full demons.
Mr. Aramaki said in this chapter that the existence of demons is "forbidden", especially approaching humans. But he calmed down right after killing the drummer demon, and showed no hostility towards Genma. So it's possible he's just enforcing a rule/law that only applies to full demons. This could also be why Genma's father appeared to be living in a prison at the end of chapter 6.
I'm guessing it's because he knew she's a second-generation demon. We were told in previous chapters that second-generation demons aren't governed by the same rules/laws as full demons.
Mr. Aramaki said in this chapter that the existence of demons is "forbidden", especially approaching humans. But he calmed down right after killing the drummer demon, and showed no hostility towards Genma. So it's possible he's just enforcing a rule/law that only applies to full demons. This could also be why Genma's father appeared to be living in a prison at the end of chapter 6.