How can I make my book even better?

Hi everyone!

So basically, I am writing a book/fanfic based on The Black Phone. 

A girl from the foster care system lives in the suburbs of Colorado in 1978 with a cereal killer murdering young boys. One day, when the killer puts his hands on her close friend (Bruce Yamada), under impulse, she decides to kill the killer. She puts a trap on him, and she hurts him repeatedly when he bites on the catch. But after his gaslight on her and a few painful flashbacks, she otherwise gets kidnapped.

I want to include Stockholm syndrome and maybe some romance (remember that Stockholm syndrome is not just falling in love with your oppressor). Also, the backstory will be revealed piece by piece, not described on the first pages (I'm now on chapter 4, and the mc just got kidnapped). I want to make the backstories a little more traumatic. Like she went through some serious problems but managed to live the most ordinary life she could. And he also went through a thorny path but decided to kill people as a PTSD mechanism. 

My question is, how can I make my book even better? Some advice, anyone? What should I avoid while writing?