Without knowing the family background it is very difficult to say.
Not all personality aspects are developed by parenting, there are other genetic, biological influences that play a part. I think in the majority of cases of criminality you will find damage done early on however it isn't always the case.
My guess is that there was drug and alcohol abuse involved leading to chemical changes and aggressive behaviour, whether or not there were also anger issues and abuse suffered during and a result of their childhood is unknown.
You would need to know more information.