It’s not really a problem. Frierens demons are explicitly monsters, not in the typical “different species” sense. They gained human language in order to better kill humans, Himmel tries to give a “child” demon a chance and she kills a family.

I love me my “redeemable monster” types, that’s why we have stuff like Tensura and other shows where monsters are not inherently bad. In Frieren they are monsters, they exist to kill people end of story. I rather like the simplicity to be honest, it’s boring seeing Demon/Monster redemption arcs all the time.