This story about immortality, grief, and the importance of emotional connections is interrupted by the presence of blunt, strawman villains who exist not as characters but as plot devices to show the “humanity” of the protagonists.
But they are quite complex. The article seems to be saying that they're just evil for the sake of evil, but they're not.
Do they need to be more human in order to have complexity?
It would instead make the story more bland imo if everyone were a nice guy with feelings.
Rather I like the way the show depicts these demons as forces of nature that don't care for silly human emotions. Even someone like Himmel let his guard down against them because he too is human and makes mistakes.