I think he quits because he's pretty tired of all the bullshit, the never-ending loop and being forced to play the same role over and over again.
Basically, Deus World is like a giant theater where the Higher Gods watch the characters repeat the same cycle for thousands of years. It's essentially a scripted world: a Hero is chosen, the Demon King awakens, and the Hero is destined to kill the Demon King. Afterward, the cycle resets, and they are reborn to repeat the same story again and again. But Deus woke up 20 years earlier than he was supposed to. Instead of following the script, he trained, and formed a bond with the future Hero, the very person who is supposed to kill him in that cycle. That creates a kind of glitch in the system because their relationship and actions are no longer following the predetermined script. So, all I can say is that our MC and Zeke won't actually fight each other in the end. Instead, they'll eventually team up and go after the real enemies, the Higher Gods who have been controlling the entire cycle.
Spoilers pls!! Pretty sure he quit being the demon lord cuz he realizes hes gonna die.
I wonder how he's gonna end though and who the main enemies are.