Skip the flashbacks. That IS the story.
Instead of flashbacks, just consider a story (trilogy or not) that shows present-tense scenes with time-skips between chapters showing the transformation from an innocent child, wanting to play, with an imagination, to nihilistic jerk.
- Abuse and neglect by parents, crying and being laughed at by parents and left alone,
- Two years later: Fending for himself on the streets.
- A year later: Fighting over garbage for food, losing.
- A year later: Stealing food, getting caught, escaping by causing injury.
- Two years later: Being "adopted" by a gang leader he trusts.
And so on. Show us how this nihilism develops, not through flashbacks but through the key scenes in its development so we readers must wonder what decisions the MC will make (always bad ones, but ones we understand as providing the most short term gain).
You must find a key scene early on to open with, showing the innocence and potential for this child, something the MC can remember and fail at trying to suppress. That is who the readers can root for; a time when the MC loved and was loved.
Don't tell us that, don't introduce a tragic or ruthless character and THEN try to redeem him. It didn't work for Darth Vader, and it won't work for you!
Personally, if I was writing this (I won't) I would not have abusive parents, but adoptive parents. I would start the character with a loving mother in a loving environment, taken away. She is killed in a bank robbery, or car accident. Perhaps dies horrifically in front of our MC; beginning his trauma.
He gets taken in by a married uncle; but not a nice person, taking him in because the MC's mother left, through insurance and lawyers, a trust fund entitling the Uncle to some regular payment of $100's per month to help with the MC's education and support. But screw that! He's a compulsive gambler, his wife is a drug addict, they put the kid in a corner and ignore him. The lawyers administering the trust shirk their duty to the kid and just keep sending out the checks.
If you want a trilogy, the first book is the first Act, and by the end of this book your character should be committed to beginning their journey, from their normal [horrific] world into new territory.
So my story arc within the first book is
- Act I, 30%: Loving relationship young child; mother and child alone with no father mentioned (or father heroically dead, a soldier perhaps), End of Act I: mother is slaughtered in front of him, and what happens next?
- Act II, 40%: Adoption, escalating horrors of neglect, abuse, living like an animal but growing into an analytic nihilist, gaining success through not caring. End of Act II: The worst step, perhaps murder (perhaps of adoptive parents), a catalyst. The solidification of Nihilism.
- Act III: A conflict here that propels a determination to change; this is the end of ACT I for the series. In this novel we established the "normal" world for the MC and why. We are sympathetic from the start and understand why he is the way he is; now we need a reason for him to exert his will and try to change: The memory of his mother, perhaps. But at the end of Trilogy Act I, we show an MC that has made a decision to embark on a journey (emotional, physical, or both) away from his normal world because he wants something better. In this case, his normal world may be just brutality and horror that he sees somehow will end in his own pointless death, and it turns out he does care.
Throughout the novel, this Act III catalyst must be foreshadowed with hints that the MC does, subconsciously, care, and uses his nihilism to excuse that caring because what he does, doesn't matter, nothing matters, so if he shows mercy or cruelty is meaningless, and making an effort to be cruel is a waste of energy. In particular, these incidents should relate to whatever happy memories he has of childhood, before his mother was killed. Even if the MC doesn't realize it, the reader sees the MC is not going to visit upon others his own horror: He finds excuse to not harm children or their mothers because he doesn't feel like it. He finds excuse to harm those that do, because he does feel like it, and nothing matters anyway. These don't have to be glaringly obvious or that frequent, but in most chapters this kind of foreshadowing should be present; if you are a discovery writer and see a chance to engineer such a scene, do it. A nihilist MC can be particularly non-introspective, his explanation for everything he does can be just that he does whatever he feels like in the moment because if nothing matters, why the hell not?
But the reader can pick up on it, that he's in denial of his feelings, and these acts of kindness or mercy do matter to him. In fact, they could matter: One of these acts of mercy can have plot ramifications that lead him into the finale for this novel, and his desire to change. (A major conflict with those around him or his environment or law enforcement or whatever, all to be realized in the next novel.)