I haven't gone through this whole thread, but has there been any talk of the Beasts directly opposing the Old Ones?
I'm not entirely sure the 'plague of beasts' is 'evil', for a lot of reasons. Not least of which being the Church wants Hunts for beasts, and the church is utterly corrupt.
There's also the inverse relationship between Insight and Beasthood - the greater your Insight to the cosmic horrors, the weaker your primal Beasthood.
edit: Even Djura agrees, and he seemingly managed to escape the nightmare/dream.
"Yes, very good. I no longer dream, but I was once a hunter, too."
"There's nothing more horrific than a hunt. In case you've failed to realize..."
"The things you hunt, they're not beasts. They're people."
"I should think you still have dreams? Well, next time you dream, give some thought to the hunt, and its purpose."
I saw it suggested elsewhere that the Moon, which is probably synonymous with Odeon, is a pagan Old One (think natural pantheon versus invasive pantheon) in opposition to the extra-dimensional horrors encroaching upon Yharnam. Some of the language in item descriptions references the Moon beating or imprisoning these Old Ones underground, which I think is important because it implies there are different orders and types of blood people are communing with.
(Look at those towers in the distance, whats sitting on the hill, and that "alien"-looking thing coming from the water ...)
The Moon, in tarot, is typically read as a symbol of the unseen forces at work in the world around us; dreams, imagination, subconscious, mystery and our inner nature. When the Moon is full, this unseen world, previously shrouded in darkness, is illuminated for us as it guides us through.
Think about Yharnam -- how much harder would it be to navigate the Night of the Hunt without a full moon in the sky to light the streets? The Moon not only guides Hunters (through the Hunter's Dream) psychically, but literally too. This would imply that it is curating the ensuing chaos to pursue its own ends, whatever those may be.
To your original point, I do not believe the plague of beasts is evil either. I think "beast" implies natural world and earthly connection, which is rooted in our own nature (and DNA/blood...) which would be seen as impure in the eyes of a church who aspires to worship celestial influence. The word "beast" sounds loaded to me, and evokes the same language as "savage" or "primitive" when used to refer to those, animal or human, who live outside of the modern, dominant culture.
Throughout the game there are two types (simplifying, of course) of enemies you encounter: "beasts" and "aliens". I use alien in the outsider sense of the word, though from a visual language standpoint this manifests as anything from spiders (multiple eyes being a metaphor for sight beyond sight) to amphibious looking creatures (water is referenced as a natural barrier separating us from the Old Ones) all the way to actual tentacled-Lovecraftian Old Ones.
The conflict of the game is between the influence of where we came from (natural influence and the physical world) and where Yharnam is headed (celestial influence and the psychic world). Clearly Yharnam worships the latter, the celestial and spiritual, as evidenced by the kinds of statues they leave lying around.
Is the hunt the natural order of evolution as we purge the old blood for the new?
Is it a means of killing off those who embrace the physical world instead of the spiritual?
Is it a means of controlling and shaping the population of Yharnam?
Is it a means of keeping the chaotic influence of otherwordly blood in check?
Is it a means to Odeon's/The Moon's own ends?
I'd say yes, to all of the above.