I'm working on my first novel, which revolves around revenge and violent cycles of human nature for more than 1.5 years. While working on some sections of the book, I feel like how do I add more depth to various issues related to violence that adds a level of depth in the protagonist's characterization. Because sometimes, I feel like the story is bland in some sections and feels incomplete or too basic. As this is my first novel, I'm having a hard time getting the story and characters right; it's kind of a nightmare.
I propose a two-pronged approach.
The first prong is research. To understand what makes your characters tick, research what violence does to real-life people. You haven't specified what sort of violence you write about, but let's take domestic abuse as an example. You could read scientific literature on the subject, talk to survivors (if they're willing), or perhaps find a support group on the internet.
The book is hardly comprehensive on the subject of how experiencing violence shapes people (there's probably not enough paper in the world), but as an introduction and useful point of reference I can recommend The Emotional Wound Thesaurus.
The second prong is to have an honest conversation with your characters. You've been writing for well over a year. I imagine you know the events of your plot by heart, and also where character actions feel contrived and emotions fake. Print your work and highlight the suspect bits, then ask your characters why they acted or felt the way they did.
You'll likely learn something about their backstory you didn't know yet. Why did the girl not run away from her abusive boyfriend? Maybe when she grew up, her father was exactly like him and she doesn't know any better relationships aren't supposed to be like that. Or perhaps your character snaps back at you. No, I did run away. You must have misheard when writing the first draft.
Either way, depending on the answer you can correct the story by adding in relevant backstory or by changing the flow of events.
How you add more depth to various issues related to violence that adds a level of depth in the protagonist's characterization depends first on re-phrasing passages like that.
"How you add more depth to… what adds a level of depth" is through clarification or simplification, not any special technique.
If your story is bland in some sections and feels incomplete or too basic, embellish those sections…
Whether your priority is embellishing those sections, or writing the other sections, isn't even about writing; it's purely a matter of working practice.
Please accept, this being your first novel is not why you're having a hard time getting the story or characters right.
When you're having a hard time getting the story right, where does it go wrong and what has that to do with your writing skills?
When you're having a hard time getting the characters right, where do they go wrong? How is that different from the way characters are presented by your own four or five most-respected authors?
(Can you explain what difference it might make that you're working on my first novel, or one which revolves around revenge and violent cycles of human nature for any given time or - come to that - which sections of your book you're working on?)