Basic premise of the story is that in a world full of sorrow, Millenium Earl – who is essentially the Devil and probably one of most terrifying antagonists in any manga – allows grieving, despairing individuals to sign a "pact with the devil" in order to revive their loved ones. But this is not actually resurrection: soul of the deceased is called into a metalic skeleton, the akuma (Japanese for a "demon"), which is then used to imprison the soul and use it as a fuel. Akuma then murders the person who called the soul back, and wears its corpse as a disguise until evolving from Level 0 into Level 1.
In essence, Akuma is made of three things: tragedy, despair and hatred, and all these things also very well represent the Earl himself. A person who has experienced tragedy may give into despair, which then leads to them hating God for "causing" the tragedy. And it is in that moment that the Earl appears and offers his contract.
Allen Walker is the best protagonist in manga, for many reasons.
First and most obvious is that he is one of few shounen protagonists with common sense. Unlike Midoriya from My Hero Academia, Eren from Attack on Titan, Natsu from Fairy Tail or a slew of other shounen protagonists, he is generally level-headed, practical, and if necessary, underhanded – as can be seen below:
Questions exist about Nea's personality and his potential hate for humanity. In fact, D.Gray Man Wiki itself claims that Nea hates humans and prefers to act on his own. But even if Nea may hate humans, this does not – regardless of what individuals in the Order may claim – mean that he hates humanity as such and wants to see it destroyed.
In fact, Nea has – few times he had been fully conscious – actually shown kindness or at least care for humans, even those who were technically his enemies. The most obvious example is perhaps when he tried to save the Finders who had been sent to capture him.