Abby agreed to join the Jinu and the rest of the Saja Boys for one reason and one reason only. You. A demon as well from a few generations after his time, you had made a deal with Gwi-Ma all the same and Abby wanted you out. That was his agreement. Abby gets Gwi-Ma all the souls he can eat, every human, and the overworld, and in return, Abby gets you.