|Horror stories are hard to write, and they are even harder to review. So I'm going to open with a caveat that I'll be repeating several times. You shouldn't take this as a critique, but as a data point.
I know of know of no formulas -- or even vague guidelines -- for how a horror story should be structured; never mind trying to generalize about the atmosphere they are supposed to create. So when I confess that "Love You to Heaven and Back" left me dissatisfied, it's not because I have strong opinions on the way such things should be written. The most I can do is try to gesture at what troubles me about it, and the nearest I can come to explaining myself is by pointing to the way the story reveals itself as it goes along.
In terms of atmosphere, it begins with gloom and melancholy, deepens into horror, and ends in despair. I assume this is quite deliberate, and in the abstract I can understand the temptation to structure a story in such a way -- it certainly increases the tension as nastiness piles on nastiness. If this is what you wanted, I can tell you it was successful. But -- and again, this isn't a criticism, only a report by one reader -- it didn't entertain me. I don't mind a nasty ending. Some of the best stories out there come with sticky ends, and I've read some marvelous stories that end even more bleakly than this one. (Like Lawrence Watt-Evans's story "Dead Babies," which is about exactly what you'd think a story with that title would be about.) But this story's unrelieved descent merely left me unhappy.
Possibly this is because the story moves in a straight line, with each awful revelation piled upon an earlier awful revelation. There is no retreat, no relaxation, and there isn't even a hint of mystery. There is no hint, for instance, that there was anything mysterious about the killings until the monster shows up, and there is no hint that there is any secret behind the kind of thing the monsters are until the very final paragraph lands on us. It's like being hit and hit and hit again.
The overall effect is very "purple" -- not in terms of prose, which is for the most part admirably restrained -- but in terms of incident. Atmosphere depends upon contrast, and so stories need to contain contrasting moods in order for the governing emotion to register strongly. Without contrasts, a story will likely come off like it's just screaming at you. If a story is already unpleasant -- because of its incidents -- the screaming will just make it worse.
I do like the idea of "Love You to Heaven and Back" -- the mystery of a murder, the horror of the facts, and the black irony of the final reveal. I don't object to any of these singly, nor even together. Actually, I think one reason I find this story so hard to review is that it contains nothing intrinsically objectionable. Rather, it is objectionable because of what it is missing -- variety and temporary relief from the tension. If I were to be presumptuous, I'd suggest that it would be better if it were longer and introduced episodes to break up the grim atmosphere. Show us the scene and mystery of the murder; withdraw to another locale and tease us with red herrings about the cause; gradually hint at before revealing the occult monster; send tantalize the heroine with a possible way of banishing the thing, and send her on a quest to do just that; then close with the final reveal that withdraws that hope.
But as I said at the top: Take this as only one person's reaction.