I assume that a lot of the suffering and pain caused around in the world, like the ones that Belpeghor showed to the kid are indeed caused by the demons themselves and by its followers. This is because the more sin there is, the more power the high demon lord has (greed, lust and so on), thus the higher the chance for the respective demon lord to appear in the physical world and wreck havoc.
If one looks at this manga objectively, it is obvious that there is clearly an oversterotypied versione of Christianity. As per usual in japanese series. Thus we can infer that the author really does not care for consistency in the characters, like demons being evil all the time for example. Japanese series, since a long time ago, have always in some way depicted demons or similar creatures as merely another fantasy race with good natured people and so on so forth, all of this in an attempt to be unique and be different from other stories which show demons as evil as per their definition.
Funnily enough it is not unique at all, and is probably one of the most overused theme (the reversal expectation of morality, for lack of better words).
Now, i don't say that this does not create interesting stories sometimes. However it gets stale very fast. Which is one of the reasons as to why people are starting to appreciate more older villains from movies in which they were simply evil for the sake of evil. They don't hide behind a veil of greyness, but they are openly evil and they revel in it.
I fear that this manga missed the point. And i think that it is due to the current manga industry, which favors the same content, or if not the content the same "narrative". In this case the narrative is that "aktually" demons and witches are just misunderstood (please ignore the 100000 souls sacrified to the Dark Master).
As for God, probably the author won't even show one thing. Not even an Angel, even tough the Priest Boy's powers are directly given from Gabriel if i recall.
I'm curious on how it will end and where the author wants to lead Mr priest boy.
I would say that this manga is going for the demons being 'alien' more than 'evil'.
The most 'evil' of the Demon Lords are currently Mammon and Beelzebub. Mammon is evil because he is entirely based on 'dominating' others through greed and a pure 'might makes right' mentality. So he fosters conflict and seeks to prove himself as the top dog by constantly creating chaos to place himself at top. He is not overly cruel, but he certainly doesn't care about the suffering of those aren't subservient to him.
Meanwhile, Beelzebub is acting purely on his own barest instincts. He only seeks to enjoy himself via consuming other live beings, mostly eating humans. He also enjoys frivolous torture and making others miserable as part of his feasting, so he is easily the one that is just pure evil.
Leviathan is the most relatable one. She has just been given the role to give birth to monsters and also an impossibly large and strong body by God. She is warped in that the world is not made for her, for she is too strong and she gives births to disasters, and her jealousy pushes her to still try to interact with people, even if it inevitably ends up with her casually destroying entire cities.
Asmodeus is a tamer version of Beelzebub. Mostly because she is more personal. Beelzebub will happily indulge in dragging entire towns, cities or even the whole world into his dinner table. Asmodeus is happy just defiling people personally. She seems to want to do that with even those she loves. But she seems to do it in a small scale, even before fighting Father-kun, she was just with a group of succubi doing her thing on a small brothel.
Belephor seems to be the closest one to normal human thought, given that he was able to live with humans. But again, he is twisted in that he is sloth, and he always takes the easy way out. Which gets magnified when you consider that he is still a Demon Lord who can level cities and if he is left unchecked he will drag the entire world to whatever twisted plan he gets into. He doesn't hate humanity, but he feels that life isn't worth it so he will care for you.
We don't know about yet about Lucifer, he seems to be trapped in ice. And Satan seems to be calm, but he is also hinted to be the most dangerous of all and at several times it is clear that his calm smile is mostly a façade.
Another interesting point is that the demons seem to be choosing to remain in sin on their own volition. Asmodeous wing was purified and returned to her previous angel status by Father-kun's light, but she still choose to embrace sin and keep her mentality of living through forcing herself on others. Beelzebub also got himself purified, but just used that power to keep indulging on himself, so he still got sent back to hell.
I think the manga is doing fine in showcasing that while not all of them are pure evil, they are incredibly dangerous and they need to be fought off for the preservation of humanity. Some may be pitable, but they are still monsters that need to be put down, because of their nature.
Witches are clearly the 'outcasts' group. They seem to be made out of victims, which is pretty representative of what the actual witches have been through history. But they are also a group set on reshaping their world into their own twisted morality and clearly ones that also need to be stopped, even if they have been wronged in the past. So no real complains here either.
Also, we saw angels in both Asmodeus and Beelzebub's attack twenty years ago.