Demon lord's death

So, im re-reading the manhua, i found out that in the last life, the dark guild fleed to the prison realm, nie li never found the demon lord in his past life, could it be that the old man in the realm just killed everyone? like, the demon lord did vandalize the monument of kong ming, that may have infuriated the old man

Also, what yall think of the demon lord? guy is really mysterious