You know from my perspective that purple mist didn't look like it was doing anything, and it really didn't look like it was going to attacker her. Besides If you're going to show a threatening mist at least make it look like it is actually going to do something, because from the readers perspective it will just seem like it was minding its own misty buisness
It wasn't attacking her. But Lunar believed it was because she's the only one that can see it with those eyes that she's gain.
So just because she can see something she thinks it's a threat? isn't that kind of like rushing to assumptions that all dead people are evil based merely on the fact you can see dead people? Isn't there good purple mist, or is there an explanation coming up that will explain why the purple mist is dangerous?
Oh and it's based not only on what Hasbro said but also on a story called the miasma by Nimaru
There are good purple mists, the problem is that they are part of a species that took over Princess Luna's body and turned her into nightmare moon. Anyone in equestria would believe it's a threat due to what it's ancestors did. But Sparking (another of my OCs) will be revealing something about them soon.
Oh, my! It's - me!