I don't know why anyone didn't post a solution for this achievement, but here it goes what I did when it unlocked.
If you are playing Vampyr now that is available on Game Pass, just like me, I hope this solution may be helpful.
Once you're near the end of the Chapter 02, you will have to go to Whitechapel and find Nurse Dorothy. After completing some few quests in Whitechapel, you'll be able to find Dorothy.
Once you've found her, you both will start discussing about her being a blackmailer. Once you're done talking, you will be given three choices in order to decide Dorothy's fate.
Spare, Charm and Embrace will be the options given to you.
I chose to spare her and the achievement unlocked. (I don't know if this achievement is missable or story-related. I think it's missable, for lots of my friends had played this game on the release date, finished it and still don't have it).
By the way, if you're hunting the ''Not Even Once'' achievement, you must spare her.