A vampire hunter gets bitten by and turned into a vampire on the job. Oops, well they knew the risks. All vampires, no matter how 'human' they start off still trying to act, all devolve into utterly feral, dangerous monsters eventually. They were taught this. It's time to accept their fate at the stake, for the good of the world.
Except... the guild/ they work for has decided something. maybe because there are the first hunter to be turned in this particularly manner, in this convenient way, or some other plot-convenient reason, but they tell the newly turned vampire that they can continue working for them. Not like they did before as a hunter in the field, of course. They've left the possibility of continuing that route behind the moment the fangs slid through their neck and the first wave of bloodlust itched up their throat. But... perhaps they'll be useful as an example to teach students the trade of vampire hunting at vampire hunter academy or wherever aspiring hunters first learn the skills to capture and kill those bloodsucking monsters terrorizing the Earth. Their willing cooperation will lead to miles better results than using feral, undocile vamps as part of hands-on training for beginners.
Imagine taking a starting class on vampire hunting and the instructor is introducing all the tools used and inviting you to practice securing a muzzle onto a real vampire. And the vampire itself is obediently standing still while also coaxing you and congratulating you and even offering tips as you slowly lock the muzzle over their face and making sure their fangs are completely barred in, not that they're trying to bite you right now. Of course, they're still chained in these classes just in case and they're still not a person anymore, this is just the organization's way of letting them do a little final good for the world in their short unlife before they eventually predictably fall down the slippery slope of bloodlust and need to be destroyed. They should be grateful for this second chance! And they are! Even when none of their former coworkers look at them normally anymore, or when they're treated as a tamed pet in the classes despite being told they're still technically an employee, or when their boss tells them that they're going to be used in teaching classes on how to put silver muzzles on vamps now, the kind that burns their skin upon contact, or that they're gonna be used in a demonstration on how long a vamp can stand being in the sun for, or how to defang one, or the limit of stabbing one before they're unable to heal immediately. The kind that's a kind of torture now, but of course they're still going to agree. Because, of course they should be grateful for any chance to continue advancing the noble cause - being part of teaching the next monster-hunting generation - even after they're now a ravenous monster themselves, and refusing any of these charitable opportunities is just proving they are indeed now too far gone, too consumed by the selfish slavering beast who no longer cares about their once human ideals, and they need to be killed for good because they're just like all the rest now.
