Might be helpful if I share my experiences.
My husband was physically verbally and emotionally abused until he escaped from his step father at 16 by joining the military. He went on to have a number of highly volatile and dysfunctional relationships and a marriage where he was repeatedly cheated on and in which (I have no doubt) treated his partners very badly in many ways.
Sometimes people who grew up in abusive families do not learn what "normal" is. Their visceral and automatic reactions to situations subconsciously learned in childhood are modeled on what they saw growing up. It takes years, effort and often therapy to identify these issues and try to remodel the underlying thought processes which have been warped by horrific experiences.
My husband used to shout a lot. Because shouting for him was not just normal but actually "mild". Well it is when compared to beating someone until they bleed. He was highly defensive. He was jealous and paranoid. He believed heavy drinking until comatose was acceptable normal behavior for a night out. He thought getting into fights was completely ok as long as the other bloke started it / deserved it. His method of arguing / dealing with stress and conflict was to smash things (his own) and storm off.
It took him years of work and self analysis plus finding the right anti anxiety medication to be able to redefine normal an acceptable parameters of behavior. To reset his benchmarks if you like.
When I met him and he told me his history, having had a very happy and well balanced childhood and youth myself with a close and loving family I was immediately touched and outraged on his behalf. I determined that I would do my best to "make it up to him", that life owed him so love, beauty and kindness to balance out the cruelty he had suffered and it was my self appointed mission to be the conveyor of all things wonderful to make him happy.
It is of course not that easy.
Fortunately I was old enough by the time we met to quite quickly realize that if he wanted to be happy only HE could make that happen. What I could and did provide was as consistent and stable a background as possible for him to thrash around on while he worked through his issues. That was not easy. There were many times when he was aggressive, unreasonable, vitriolic, verbally abusive, emotionally unstable, untrustworthy, deceitful and all manner of other things. I recall lying in bed while he ranted and raved and punched walls and claimed I didn't live him, no one could, I would cheat like all the rest etc. and just responding to those temper tantrums calmly with "I love you. I am here for you.". Just those words repeated regardless of what insanity he came out with. Sometimes I couldn't stay calm. Sometimes I cried. Often I was scared. Many times I felt resentful. Occasionally I felt that it was just too much trouble to be worthwhile. But it seemed everyone else had given up on him so I gritted my teeth and I vowed I WOULDN'T.
But I stuck it out because he TRIED. He went to the doctors. He changed his environment and modified his unhealthy behaviors. He cut down alcohol. He went to therapy. He took his medication as directed. And month by month, with the occasional regression he got better. He healed himself as best he could. It took a year for the biggest improvement. 2 more until the bad times were so infrequent you forgot about one before there was another. Then one day you realize It's been 6 months since you thought about walking on eggshells. And the last time you had a row he was the one being reasonable and thoughtful compared to you. And he seems happy almost every day, even when he is tired and stressed and the cat has scratched the toddler and the baby woke at five am and you're late paying the electric bill. And it was worth it.
But my husband was in his 30s. Had I met him in his 20s I would never have been able to stay with him. By that age he had gone through the roller coaster of highs lows and emotional and physical traumas his abusive past had seeded. He was ready to try to make changes and to try to get better. He was mature enough for the introspection required to see the damage inside him which was spilling out and poisoning his life. He has said to me he would never have been able to see that at 20.
Of course everyone is different. Perhaps your guy is very self aware. Perhaps his experiences affected him in a different way to my DH. I dot know. All I can say is in MY experience you need to be prepared to have very firm personal boundaries, the strength not to be easily hurt or hold grudges and an awful lot of patience and compassion. It's hard to empathize with someone's internal hurts without condoning or enabling their acting out. It's hard to provide a model for "normal" when you get sucked into someone else's alternate reality. It takes a lot of self belief not to get lost in their problems.
But it can be worth it.