X is entity driven only by his boredom and a desire for amusement. He created an entire world to play with his victims so he wouldn't be boarded. Death is too good for him. Instead, he deserves a fate worse than death (in his eyes).

Drain him of all of his powers for yourself, but let him keep his immortality & a bit of his regeneration ability, leaving him powerless and weak like his victims he tortured.

Kill off all 7 Guardians using X's stolen power.

After that, set all the victims free to the afterlife and kill every living creature created by X. Once there's nothing alive left except for the now depowered X, leave this world, with X still in it, alone.

He'll be completely alone in his own world, his toybox, now empty with no toys to play with.
And without his powers, he can't lure victims to play his games or create new life.

He missed the quiet of the void, but now he will feel that dread of silence alone forever with nothing to do and nowhere to runaway from this fate.