The night stank of evil. Of rotted sin and soiled depravity.
It rose in the steam from the ice-covered sewers and oozed from the shadowed alleyways. Had Alexander Gedrick still been human, he wouldn’t have noticed it. Now the scent was fine perfume; the perfect concoction of fear and despair heightened only by notes of hate and rage.
Okay, the fragment: If you had a lot of fragments, I'd probably edit a bunch of them out (because if there are a lot, they're annoying, because they mess up rhythm, because they're wrong and make the narrative staccato and without much cause/effect relationship between parts), but if you use them only once in awhile, I am much more likely to assume that you want that specific effect this specific moment and let it stand.
I don't actually much like this fragment, because when I read the sentence aloud, the fragment really sticks out as incomplete. I'd probably rather put "smell" or something in the fragment so that it doesn't come off so much like just an afterthought that should be connected to the sentence. But it's okay. The wording is very nice.
It rose in the steam from the ice-covered sewers and oozed from the shadowed alleyways.
Here's where the problem starts. What does "it" refer back to? You're making the reader create a noun antecedent for that pronoun, or guess at one. What can "it" refer to? The night. (No.) Evil, sin, depravity? Maybe, but that would be "they". I suspect that's why I was mentally inserting "smell" into the fragment, to give a noun for "it" to replace.
Well, there isn't one. So I'd replace "it" with the actual noun. What's that? The rotting smell? OR you could go with Its steam rose from....
Had Alexander Gedrick still been human, he wouldn’t have noticed it.
The inverted construction (Had) makes me think that this is probably a bit more formal a voice, and that's fine. I don't like the "Dolly Parton" feel of that sentence (top-heavy, the first element longer than the second), but I might fix that by lengthening the second clause-- he would have walked by the alley without noticing, or he would never have noticed, or something like that. Use that as an opportunity to put us in the scene physically with another slight reference to the setting, maybe. And you know, if I wanted to kiss my darlings, I might follow through with the inversion, moving "still" to the end-- a nice little Victorian touch which might be fun with the whole evil thing:
Had Alexander Gedrick been human still,
Notice that a more contemporary feel might lead to a simpler line:
Back when AG had been human, he ....
But the formality is a nice contrast with the alleyway and evil and stench.
Now the scent was fine perfume; the perfect concoction of fear and despair heightened only by notes of hate and rage.
I am NOT one of those semicolon haters (and you know who you are :). But this semicolon is grammatically wrong. It should be a colon. A semicolon is a connector, a replacement for a comma and conjunction to connect two clauses. But here you have one clause and an elaboration, and elaborations take a colon (in more formal paragraphs) or a dash or comma (informal).
I also like modifiers, so when I say "too much," it's probably WAY too much. I think "fine" can and should be deleted so that it doesn't diminish the "perfect".
Now the scent was perfume: the perfect concoction of fear and despair heightened only by notes of hate and rage.
Could be... was LIKE perfume, or perfume to him, but those both clunkicize this, and you want it to feel streamlined and poetic. Oh, I don't know if you need "only". I like the "notes" because it sounds like how they describe wine... "notes of licorice and black pepper."
So streamline-- let the elegant bones show. But precision is all in such a formal paragraph. So "it" there in the second sentence is going to throw the reader off where a noun wouldn't.