Ah, I see... it might be, then, that just the fact that they were fairies made them unable to be destroyed. I guess that even if the Lunarians purifying weapons killed them, they'd just respawn. Or they were so impure that the Lunarians couldn't get close, as you said. Or a bit of both.
I don't think the Lunarians themselves would have a serious problem when faced with impurity, rather it's something that
must not affect the Lunar capital. Just saying, but this is not the first time fairies were brought to the Moon : in SSiB, Remi's fairy maids were brought on the Moon also, and Yorihime had no problem being in their vicinity. If we don't give "pure" a holy-like connotation, "pure" is basically the 100%, the wholesome, the homogenous, the untainted ; therefore "pure" does not admit change (say for example a plebeian marrying into a royal bloodline : the result of the union won't be as "pure" as the bloodline) ; "impure" is the mish-mash of various things and admit change, it can include things that have nothing to do with the original thing, it admit change (IMO) ; "life" necessarily imply "death", they come together : Lunarians deem "life" impure because of that : for example, the Udonge flower is a flower that only grow when tainted with "impurity", meaning that this flower being in the lunar capital makes no sense since it can't grow there, yet there it is, used as an impurity detector of sorts. The Lunarians's fear of impurity is actually pretty legitimate because if "life" tainted the Lunar capital, life and death cycle would apply, and might threaten the long, presumably prosperous lifestyle of the Lunarians (I think that is also why Yorihime found Reimu's corruption attack in SSiB more threatening than the rest).
Anyway, my point is that the life-force is what they considered as impure. Regardless, fairies are still life-force whether it gets boosted or not. I assume it might as well be radiation for Lunarians ? Still, Yorihime and Toyohime have been on Earth (In the Bougetsuhou 4koma), which is filled with life-force / impurity (I?d rather use ?kegare? for more religious stuff cuz confusions) and they were pretty fine there (plus the fairy maids earlier).
What I meant to say is that impurity is not instakill for Lunarians, they just don?t want it in their Capital. Also, it should be pretty silly of the Moon to design weapons that are ineffective against the very thing they hate the most ; their weapons should affect (as in ?kill?) fairies but won?t do anything toward the lifeforce they bring with them. The Lunarian?s purification (not Junko?s) might just be a sugarcoated way of saying mass murder, the kind which kill the land so dead even the grass won?t grow anymore. Damn fancy words?
I guess there?s gonna be a lot of confusion on the kind of definition of ?pure? and ?impure? that are going to be used this time, but there is certainly more than one.
On an unrelated note, the ending is fairly more accessible for this one, despite the difficulty. I take it it?s gonna extend into something else ? I mean, c?mon it?s ZUN?s oh so beloved Moon saga ! :V This can't be the last we hear of this Sagume person
Honestly, Sagume is almost, dare I say it, a Mary Sue
... I think you misspelled "Lunarian" :V