The Denarian thing is *very* weird.
Two ways I can see to look at it:
1) in borderline cases it depends on the wizard's belief. Killing with magic is tainting at least partially because it makes you, deeply, the kind of person who *would* kill another person. So if you're thinking of the Denarian, loup-garou, White Court vampire as a monster, there's some insulation. But wizards aren't going to think of other wizards as less human, so they are just as tainting to kill as ordinary mortals. In this view, if the wizard knew the were-creature was a human practitioner, it would count as human, but if they just killed a werewolf that jumped them in wolf-form, it might well not.
2) Wizards & other practitioners are 'more human' than White Court vampires, Denarians, other humans-with-magic. Unlike the others, there's no sharp line between wizard and mundane -- it seems that magic is a pretty common human ability in the DV, so weak it's irrelevant in most people, quite a few people have a touch of talent that can be more if trained, a few have serious sorcerer talents, and one in a million are Council-level. But even the strongest wizards are just an exceptional form of a trait already inherent in humanity at some level; wizards aren't a 'supernatural race'. In this case, the human-practitioner kind of werewolf that the Alphas are would count as human. [Hexenwolves might theoretically be like Denarians, at least while in beast form... not sure about them.]
The second seems closer to the feel I get from the books, personally...