Ah, Mephistopheles, the Essence of Despair...
Despair is an extremely rare breed of Infernal. Normally, Infernals represent negative emotions (for the lesser ones) and sins (for the greater ones), but Despair is something that's the result of negativity and sinning. As such, Despair is more of a "wildcard" Infernal, being essentially composed of all negative feelings and sins, and thus representative of suffering in general.

Mephistopheles had the unfortunate luck to be an incarnation of Despair, which eventually led to his nihilistic nature.
At the same time, he's the will of all who carry wickedness in their hearts, no matter how innocent and small, or how unforgiving and great.
As he's chained to the will of the people, he essentially does nothing more than execute the wishes of those who wish harm upon others.
He truly is cursed by destiny for his disposition. He genuinely wishes to improve the world, even if it's through chaotic and dark, immoral means, but slowly he does begin to realize that everything is futile, and slowly begins slipping into nihilism and insanity.

In his free time, he indulges himself to cruel and inhuman experiments on his victims, which is his way to expose to what he believes is the true nature of man. All of his merciless actions of seemingly mindless destruction and slaughter are a testament to what man is capable of, and has ever done, exposing the evil taking place in the world by the hands of the mortals.

His desire to end all of the suffering in the world eventually becomes so extreme that his despair literally eats him out from the inside, leaving nothing but a brittle husk behind, only being weakly held together by a small will of wanting to achieve his nihilism, until death itself takes over what little free will he still had, turning into the God of Death.

Luna, the Essence of Hope and thus Mephistopheles' arch nemesis, travels into the depths of his soul to find the origin of his demonhood, and manages to sever the link between his identity and his demonhood, essentially freeing him from his cruel fate, even though his remains are forever sealed away in the Realm of the Dead.
As Death and Despair can never be truly defeated as long as there are people longing for them, being sealed away from reaching the mortal realm was the only way to stop Mephistopheles from reincarnating as Despair and channeling Death again and spreading terror across the universe once more.