Not a mistake, just something that a lot of people misunderstand.
Also, I just re-read the early books, and the two instances I noticed of guns failing destructively (Greg Beckitt in Sells' house at the end of SF, and one of Bianca's goons in the confrontation at the end of GP) were in environments/situations with much more magic present than just Harry's proximity -- Sells' house was the site of a big thaumaturgical spell, and Bianca's place was the center of a major effort to thin the barrier between the Nevernever and the mortal world.
Murphy's gun doesn't mess up just because Harry is standing nearby.
Apparently, using black magic, even once with a reasonable cause (self defense) like Harry did against Justin had repercussions on Harry, but in Cold Days as he races from the wild hunt and uses magic to blast a hound the hound turns out to be a human - another use of black magic on Harry's part, but we haven't seen any effect from that.
Well, Harry killed MacFinn the loup-garou with magic back in book 2, and that doesn't seem to have counted -- because MacFinn was a monster, not a human,
right then. Harry's also killed at least one Denarian with magic, with no apparent repercussions, and he discussed that fight with Luccio afterwards; the Laws never came up. It seems that beings with human souls who are "in monster mode" at the time don't count.
Additionally, things keep getting redefined in the series as we learn more. Most wizards seem to think it would be fine to kill white court vampires with magic - no black magic twisting them because of it. But over time, we've learned they are scions (per an angel), human enough to become the winter knight, and probably human enough to wield a sword of faith. All things that point to them being human enough that it should be black magic to use magic to kill them, but we've never heard that is the case...
And Harry does kill one with magic (Madrigal Raith) with another Warden (Carlos) watching, and it's never brought up. WCV's pretty clearly do not count. Why not is a big, big open question though...
Although, Mab says
Thomas is human enough to be Winter Knight because he's in love. I don't think any random WCV could be... definitely not one like Madeline who is totally controlled by their hunger, and probably not one like Lara who is partnered with it. (I guess you could argue that Lara, sometime, could choose to live like Thomas though. I think Madeline was past that point though.)
The whole black magic thing appears subjective to the viewpoint of the caster, but then we are told repeatedly that it isn't (both in the books and in WoJ). It doesn't make sense.
Well, I think there's more than one kind of subjectivity here. It won't be non-corrupting just because you believe it's right or because you meant well. (Instead, you'll just end up doing more and more black magic to 'make things better'.) But there might be some room for subjectivity in
other areas.
IE, a wizard who really knew Thomas as a person might be corrupted by frying him, whereas another wizard who just met him as a WCV across a dueling ground wouldn't be -- because the first guy would then have to deal with "well, this guy is a pure human, but he's way more evil, why can't I just fry him?"
IMO the corruption has to do with the direct act of will required to use magic. If a faerie put a glamour over a human to make them look like an attacking Black Court Vampire, a wizard who then fried them might not necessarily be corrupted, because the act of will involved was "kill that vampire" not "kill a person".