Genetic factors can lead to violent behaviour, but from what I recall from genetics and neuroscience courses, it does not mean they will.
The “warrior gene" was mentioned and studied in the courses, it was also said most whof have it do not have a hight propensity for violence. If I am not mistaken, the "warrior gene" is more prevalent amont European men, yet, they are not more violent than other men.
Our genes alter with ambient, therefore, having the "warrior gene" alone may not mean a thing and it mostly certainly does not.
Genes also change with trauma, etc. but, again, it will all depend of a series of several factors. It is not so cut and dry. The brain also changes with a lot of things.
Both genes and brain can re-change.
If genetics and neuroscience start to be used to let criminals get away, society is in serious troubles. I suspect it has more to do with clever lawyers than genetics or neuroscience.
Sometimes good things fall apart so better things can fall together. (Marilyn Monroe)