You make these assumptions about life among the miasmics.
Well, maybe some of them are peaceful, like the ones who were part of the crew on the Goya, but what has gradually been shown, through little hints and direct examples is that they for the most part are violently contentious and territorial.
When we first encounter them they have occupied a barn-type place that they have simply modified for their use, while they carry weapons and wear skulls.
It's clear that there's history between them and humans: (Kill on sight.)
Their cruelty and vicious method leaves quite a bit to be desired for being able to get along with them- (I think they act like they're on drugs.)
I think you're tying way too hard to equate peaceful with good and violent with bad. "Violently contentious and territorial" could describe pretty much all animals; even herbivores fight for mating rights and food. Humans have a long and proud history of building weapons that make them better at being "violently contentious and territorial".
Moreover, even the "peaceful" people on board the Goya
were heavily armed, while Dang seems to have a fragmentary understanding of the Miasmic language(s) and Beautiful Flower actually showed more compassion towards Dang than her human friend. Everything we've seen so far seems to indicate humans and miasmics have normal relations with each other, which in a context of competition for rare resources mean they fight often.
The worst we've seen is the Miasmic fleet which was at Primrose and is possibly (probably?) the same fleet that opened fire on Dang and sunk the Penumbra
. Their behaviour is somewhat consistent with some kind of religious movement; but for all we know, they could simply be at war with some of the human groups (and we don't even know why—they might not be the aggressors). Their exposure of humans to the Mist may have religious or cultural significance, but since the Mist seems to confer true prophetic powers on the human victims it could also be a practical way to gather intelligence. For all we know, there are human groups out there who do the same.
With regards to their origins, there was the word "evolve" scrawled on the barn where we first saw them, which to my mind is a hint they probably evolved out of humans. In fact, that exposure to the Mist kills humans is our interpretation—the story so far hasn't actually ruled out that what we saw on Primrose was the Miasmics "recruiting". The only human we've seen that was "killed" by the Mist, IIRC, actually committed suicide and it's Dang's guess that he did so due to the Mist.
In short, I really don't think good vs. evil is a theme here at all...
With regards to the last page, was Dang even attacking with her flare gun? I know she's used it like a weapon before, but its original purpose is calling for help, and she really has nothing to gain by attacking the Miasmic fleet. Woman with flare gun versus warships with naval artillery doesn't seem like a good idea. Given her pained expression, she may have accidentally fired it, or maybe she's gritting her teeth because her only hope of survival is throwing herself on her enemy's mercy?