There's zero proof the father had a history of violent abuse. It's something Dungey said in court. There's nothing to back it up in any of the letters. What lawyers say is not evidence.
Also, as mentioned by Forest_Wood above, the father was supposedly not in Smich's life so it's reasonable to ask who abused Smich if the father wasn't around.
I personally believe both Smich and MIllard experienced serious abuse as children, which is not in any way an excuse for their crimes. Many people who are abused do not go on to murder others for thrills. But serial killers almost always come from situations where they were abused, often sexually, as children.
Smich's family's on-going tolerance of his **** lifestyle and laziness suggests to me that they felt they couldn't ask him to behave like a normal adult and become a productive member of society. Their unwillingness to specify what happened to him is pretty normal in these circumstances, where people often feel a strong sense of shame about what went on even if they were in no way responsible for it.
While Smich's father being absent didn't help the problem, I'm far from convinced he was the source of it.
Also, people seem to overlook the fact, as testified to in court, that Matthew Ward-Jackson was a friend of Smich's older sister and would be at the house to visit her.
https://twitter.com/ShannonMartinTV/status/933394165571997699