World-building: Plot holes are a lot easier to make by mistake than atmosphere for the average author. Most of the "this place sucks" seems atmospheric to me - Rowling may not have thought as poorly of her world as I do, but I doubt she thinks it'd be a great place to live after the wonder wore off.

Yet the "this place sucks" atmosphere doesn't actually kick in for real until Book 5, when the protagonist finds himself on the wrong side of the barricades for the first time (and also when Rowling leaves the teenage angst tap on). Until then, the dominant theme is that of a marvellous, whimsical magical world that's so dazzling with its uniqueness that you don't stop to question the holes and contradictions. It seems likely that touches such as Vanishing kittens are meant to be seen in this context rather than the negative one of the later books (which in any case focuses heavily on formal structures such as law, politics and media rather than day-to-day social practices).

Unambiguous evil: I disagree entirely. Yes, the Death Eaters and Dementors are unambiguous, but Snape drove back and forth across that line so many times that it's ridiculous("possible", really?), Grindelwald was appealing enough to draw Dumbledore in, Hagrid was criminally stupid half the times we saw him(literally), Lockhart/Slughorn/every politician were some combination of pathetic and loathsome, Percy Weasley was an utter git and a massive enabler, and I could go on. Admittedly, most of those weren't big-E Evil, but they certainly did not lack for human flaws and ill consequences. Don't let the unambiguousness of Voldemort or Umbridge fool you.

That's exactly my point. Apart from Snape, the reader never has to stop and think "is this person good or bad?" Grindelwald is charming but proto-evil even in his youth (based on his views), Hagrid is unambiguously well-intentioned even at his stupidest, Lockhart and Slughorn are clearly low-grade evil (though at least by the time we get to Slughorn, Rowling is learning to make bad people slightly sympathetic), and Percy Weasley has no redeeming features until he actually gets redeemed. You never have to think in order to tell good from bad (apart from Snape). And this leads me to believe that if something is not portrayed as bad in the least, then you're not meant to think it is, because it seems foolish to save all your subtlety for details of world-building and use none in characterisation.

Slaughter: It wasn't that long ago that's precisely what happened. And even today, I spent the last few days with the part of my family that's farmers, and all of them have been hunting since childhood. Perhaps that's "desensitization", but if so it's an utterly common sort in the right cultures. Death being locked away is a modern innovation, not the natural order of things.

That's not relevant in this context, though. We're not dealing with people from cultures elsewhere in the world, or from a different time period. We're dealing with modern British children, some from Muggle society and some from wizard society, engaging in practices that contradict at least the norms of Muggle society and possibly the wizard one as well.

But Vanishing is not suffering, it's simply death, as odd as that sounds. That's a lot easier to handle when it's applied to animals.

Yup, and that would certainly reduce the psychological impact of Vanishing Charm practice to some extent. Of course, there are also other spells practised on live animals that do not have this saving grace ("your pincushion still quivers in fear whenever somebody approaches it with a pin").