I think a lot about that pop culture trope where "human" is defined as Purely Human, and the second you add something else to the mix then you become Something Else.
Grow a pair of wings? No longer Human.
Weird eyes and psychic powers? Not Human Anymore.
Twelve feet tall and green? Yep, you guessed it, that thing ain't human.
The trope, of course, is rooted in disfigurmisia - and has fingers in several bigotry pies, including racism and fatphobia. Because it's not about figuring out what defines a human being: it's about getting to decide which humans are human enough.
It's no accident that this trope is often used on characters with a lot of privileged identities - male, cis, abled, well-to-do, conventionally attractive, and/or part of a dominant ethnic group. The mental image of a "human being" is usually built on these traits, and the best targets for Maximum Angst are ones who have no other experience being dehumanized.
I wouldn't say that this is an inherently bad thing. It's possible to write a story about a privileged person "falling from grace", and in losing their official humanity, exposing the flaws and dangers of this system and/or developing a much healthier framework. You can even draw attention to the fact that the "real humans" are just the biggest assholes - it's been done, and even done well.
On the other hand, it's also very easy for this trope to go very wrong. Sometimes, rather than using the character as an entry point to talk about a widespread problem, the author treats it like an individual tragedy - a fluke that's only worth caring about because it happened to This Person Specifically. The fact that this only happened because the rules of society allowed it to - because we've defined "human" around such a pointlessly restricted standard - ends up floating around the story, unacknowledged but obvious in a way that betrays the author's endorsement of the system.
And then there's the other form of tacit endorsement: "no, guys, THIS time they're REALLY unfixable monsters, it's OK to shoot them." It's a popular horror variant of this trope, where - sometimes after paying lip service to the natural human urge to save people who have the Evil Virus - the movie makes a point of the fact that the former humans can't be helped.
It's not impossible to do this version well, either. I'm partial to the tragic zombies in the original Dawn of the Dead, and of course, Sinners knows exactly what the fuck it's saying with those vampires. But there's the rub: both of those movies had something specific to say, and not "eugenics is actually good if the people you target are freakish enough." In fact, both Sinners and all of Romero's zombie films make a point of refuting that mindset.
If I have a point to this post, it's that I fucking love me some transformation horror. I'm trans and disabled, so I'm gonna eat that shit up with a spoon. But also, I'm trans and disabled, so if I catch a whiff of the author implying that I deserve to go under the bus, I'm out of there. If you want to write a good story about someone losing their humanity, you have to start with a clear and meaningful definition of what you mean by both "human" and "nonhuman" - and what, specifically, you're implying when you say a character is no longer human.