>>280885712It's about growing up and recognizing what are the truly important things in life, which throughout the story is implied to be the ability to make real connections with other people. It is why the En Family was never depicted as pure antagonists, because despite everything it is easy to see that they do have a real bond with each other.
In the first place, everything gradually went to shit because Ai Coleman wanted to be a magic user thinking it will magically fix all of his problems with being born a Hole inhabitant. His real issue is extreme self-loathing. And then we are shown that Aikawa, his more mentally stable personality who actually gets to live in the magic user world, ended up just being a bum who does nothing other than goofing around all day, while the other personality is a personification of Hole itself. It can be taken to mean that Ai never actually escaped the Hole, so to speak, and the one guy he made a real connection with was killed by his own hands, again because of the Hole.
Even Chidaruma, who is practically the main source of all the conflict in the story, is depicted as a bored autist with no real friends who does nothing other than the equivalent of an omniscient being fucking around and finding out. That's why he tried to befriend Hole-kun and also why the other devils turned him into a human in the end, with the implication that a much shorter lifespan and no powers will finally teach him the true value of genuine social connections.