I actually ended up doing a little bit more reading into this and I found out that people were already aware of the ending at the beginning of this year. From what I saw, a huge majority of the fans of the light novel and later, the manga (from the raws) were really disappointed in the ending.
Apparently I wasn't the only one who saw the foreshadowing of the main character ultimately being the antagonist. There's a rumor that the author of the light novel grew tired of writing the light novel and just went a completely different direction toward the end.
Honestly I thought "is the big baddie the OP from the future or some alternate universe or something?" as well at a few points in the story. With how hidden and mysterious the big baddie was I expected some big reveal. Instead he turned out to just be a random rogue experiment from the past. A nobody we'd never heard of or seen before from the perspective of the manga.
Not only that, the entire confrontation between the big baddie and the hero started and was resolved in one chapter and a few pages. Feels like a rushed ending, so I wouldn't be surprised if the author was just done with it and quickly wrote an ending into it.
I read through this manga a few times before it ended. Maybe it was just me and I was misreading the clues, but it seemed to me that the identity of the Demon Lord with the wings was somehow the main character. Like some sort of timeline double.
When it ended with a nobody, I was more disappointed than if it it were someone else that was already an established character.
It was anticlimactic and I kind of feel like the clues sprinkled in the last 100 chapters just don't point to someone random face. I don't know. Oh well. ( ̄へ ̄)