First off, Sonju doesn't need to eat humans to retain his intelligence and humanoid appearance. But to explain why is somewhat of a spoiler:
His traveling companion Mujika, is the Evil-Blooded. Rather than explain myself, I'll quote the wiki: "Mujika is a special demon who retains the characteristics and sapience of humans without requiring to consume them. Other demons who ingest a drop of her blood gain the same abilities and are also able to spread this unique body composition to other demons."
Mujika and Sonju both follow a religion that forbids them to eat farm-raised humans. Thus, Sonju thinks it would be okay to hunt humans born in the wild, and this is something he wants to do.
I haven't seen the anime yet, which I think may tell a different story from what I've gleaned, but I've read the manga up to volume seventeen. From what I've seen, Sonju seems quite loyal to Mujika (and for good reason given her ability). Though it may be his private wish to hunt humans again, he might listen to orders from Mujika if it is her wish to help the children escape to the human world. I can't say this definitively though.
For the last point: If I understand correctly, I think the promise makes it forbidden for the demons to attack the Ratri clan, and any humans from the other world. I could be wrong. I'll try to verify this at a later time.