In their defense, making a movie for an existing franchise seldom ends well; since it has to be isolated and self contained, it's usually an okay one-off thing at best and varying shades of terrible and/or pointless at worst.
Adapting another work into a movie usually goes even worse, partially due to the differences in different mediums, partially because the isolation gap is basically absolute, and probably in large part because existing fans of the franchise are a relatively captive audience, so you just don't get as much benefit making it how they want compared to trying to rope in outside parties.
When you combine these two in order to adapt a long-running series into a standalone hour and a half, you're basically guaranteed to fail. I've even seen this happen
retroactively, where a TV series based on some movies just had so much more time and thought to put into developing everything that it made the base works look rushed and bad.
...though to be fair, the movies in question were the Star Wars prequels, so the bar may have been a tad low.
EDIT:
I swear to God, after this manga reaches its ultimate conclusion, Skull Knight better get his own freaking series.
A lot of it is implied in little things like "hello struggler" and the cryptic warnings about "not having more than one you care for" in the context of Schierke's master and his friendship, plus how he responded to finding out she brought the armor out.
It is telling, I think, that he targets Void first.
I forgot to mention that I LOVE little details like this. Faint insinuations of terrible, world-shattering things long forgotten are the best.