Songbird in the Gallows by Alta Hensley

Murder is their love language…
Five years ago, Saylor Mitchell’s father was brutally murdered in front of her, shattering her world. Now his killers have tracked her down to finish the job. She is unexpectedly saved by her father’s closest friend, the enigmatic Blue. Saylor's savior—and captor— is a reformed, murder-sober assassin struggling to escape the clutches of his own violent history.
Blue’s gothic mansion, perched on the edge of a cliff overlooking a stormy sea in the town of Grimlock, is a labyrinth of secrets. And so is Grimlock, a small town whose residents all seem to have a penchant for murder that they’ve managed—mostly— to overcome.
Saylor soon realizes that Blue holds the key to avenging her father’s murder at the hands of the Crow, a gang of amoral contract killers. Only problem? Saylor has never killed before. Good thing those who can’t kill…teach.
Blue finds Saylor an apt pupil and the attraction between them morphs into a dangerous obsession. Can their love conquer Blue’s final, darkest secret?
Tropes:

* Morally grey leads
* Touch her and die
* Dad’s best friend
* Possessiveness
* Gothic mansion
* Murder mystery
* Forbidden love
* Age gap
* Forced proximity
* Anti-hero
* Dark romance
* Gothic romance
* Fairytale retelling