It doesn't go much into it but the humans started harvesting creatures for power and not sustenance. Maybe if humans were subjugated by the elves then sure it makes sense as to why elves say its bad. It just doesn't seen worth it for this show to get that deep into the right and wrongs of harm and suffering to help others. A simple blanket can suffice.
The show is too young. I think either better explanations or more questions will be given. I wouldn't be surprised if we see this later down the road:
- Why humans aren't automatically attuned to the elements
- Magical creatures who use dark magic
- Why dark magic is as bad as folks say it is
- Why humans created dark magic
- Elves are evil cause they attack Best Aunt
I just don't see how dark magic can be seen as good when the idea behind it is "kill things and use their life essence to do magic" while elemental is "connect with the element arcanuum and cast fireball." One is easily more good than the other because it doesn't require life to fuel it. Plus, more and more powerful dark magic requires more life. Viren so gonna sacrifice a city one day.
Maybe one day people all around will embrace and use dark magic for personal, politcal, or whatever way they see fit, but I suspect if that did occur on a widespread level that all life would be consumed at a very scary speed.