Evil Ghosts and weird beings are actually a type of Demon; more precisely, their power and the power of Demons share the same origin, known as "Ghost-Demon."
So the reason Evil Ghosts are undying and unkillable is because, in essence, Evil Ghosts are nothing more than an extension of Demon power.
And for some unknown reason there exists here this Hell that spawns Demons, but as long as one possesses the conceptual Demon that makes all Demons feel fear, that is enough.
This is a battle of concept versus concept; Xue Dili needs a power that, on the conceptual level, can kill Demons.
So!
"Boom!" The Explosive Bullet Gun in the Doomsday Warrior's hand effortlessly slaughtered the surrounding Demons, and he even grabbed a Demon's skull one-handed and crushed it.
This method of killing, more terrifying than any Demon, instantly made several nearby Demons feel fear; they even wanted to drop their weapons and run.
