The principle of causality says that everything has a cause. So all of our actions have a cause of one kind or another. So if there is a reason why a person steals from a bank, then would this not mean that she is not morally responsible for her embezzlement?
A difficult question of free will. One with which I admit I have trouble.
When Charles Whitman shot and killed so many people from the clock tower at the University of Texas in '66, he was subsequently found to have a brain tumour which:
‘…conceivably could have contributed to his inability to control his emotions and actions.’
Let’s say that it was a contributing factor. A medical condition that caused him to lose some control. That affected his free will choices. How much does this lessen the degree of culpability that Whitman should accept? With neuroscience a lot more advanced a half century later, it could be possible that the same actions today would result in the perpetrator being found not liable for his actions.
So if a medical condition might allow this, why not a genetic disposition as well? As there are genes that are associated with violent behaviour:
"Each criminal (in Finland) was given a profile based on their offences, categorising them into violent or non-violent. The association between genes and previous behaviour was strongest for the 78 who fitted the “extremely violent offender” profile.
This group had committed a total of 1,154 murders, manslaughters, attempted homicides or batteries. A replication group of 114 criminals had all committed at least one murder.
These all carried a low-activity version of the MAOA gene, which previous research has dubbed the “warrior gene” because of its link to aggressive behaviour".
bbc.com/news/science-environment-29760212
To what degree can possession of this gene be considered a mitigating factor? Possession of it doesn’t mean you WILL be violent, but it is definitely a contributing cause. One which is beyond the control of the individual.
If a tendency to violence makes you a less moral person, then your genetic make-up is a constituent part of your morality.