That backstory question sounds like you needed to write that scene. The trick is where to put it.
Could be that it’s something that haunts the character, as well as drives what they are doing in the story now, but remains a thing that haunts them. The best spot the backstory scene could be dropped in is somewhere after the 3/4th part in the story though because this is where what the character does hinges on whether they are going to continued to be haunted or turn the tables. Do the one thing they swore they’d never do, or froze with fear at just the thought of it in the past and now it’s the one thing that will determine the flight or fight, and at this point in the story, it will need to be “fight” that is chosen. It’s the character overcoming what has held them back that has been rooted in the backstory scene.
That make sense to you?
And I love the idea of the villain at the start of DEATH LIST. My question is, did you pitch it at a romance editor or a mystery editor. They are going to look at this entirely differently. A mystery editor is more likely to say, “great kick off scene”, where the romance editor wanted things to begin with the romantic couple and probably felt having the villain first would put romance readers off.
What works well in one genre isn’t always welcome in another genre. The thing to keep in mind is “play to your audience” so the first thing to decide is who that audience is: the romance reader or the mystery reader?
Hope these things help. And as I keep telling myself to stop dreaming up new stories because I’ll never get them all written before I die otherwise, I know exactly how starting a new story happens whether you need yet another new story or not. 🙂
Happy that the timeline idea appeals to you. It’s been very helpful to me when I need to see where I need to be heading in a part of the story. It’s one of my 60 WAYS tricks that works either for sorting out the plot or getting out of a writer’s block box canyon. 🙂