To be honest. He could have admitted to her that his actions were fully what his charges suggest and from a British POV a sentence like that seems barbaric. <modsnip>
Unfortunately though, whatever the circumstances of the original crime, growing up in the 90s in Florida's prison system is not a place where people tend to learn how to have healthy, mutually respectful, non-violent relationships.
He might grown into a damaged but decent enough man who was deeply regretful and just wanted to start over. Who found a woman in an unstable part of her own life he wanted to help and who in turn accepted his past, despite the pushback from her own family. However, he was 14/15 when he went in, if he didn't come out with PTSD, a hair trigger response to violence, with control meaning safety/security and a boatload of other untreated mental health, anger and attachment issues... well he would be one of the few.
With the best will in the world, that tends to bleed into a romantic relationship. Especially if she has her own mental health/addiction etc issues. Then add eight years of becoming more and more isolated... as (speculatively) they - in their eyes - continued to be victimised by society, social services and a system who keeps taking their babies away for a crime he did at 14.
So yeah TLDR, even without making him an explicit DV villain. Their past can seem sad and unfair, their motivation deeply sympathetic and complicated by mental health and trauma... and still the situation can be domestically violent, unstable, controlling and very dangerous for children.
Unfortunately, there is a days old baby in the middle of it and any hypothetical sympathy for their situation comes second to babies safety and health.