This is a far too complex of a subject to merely narrow down to genetics.
Socio-economic status and upbrining has a greater influence on aggressive behaviour and violence.
Here,
Here,
Here
The article itself says "
A breakthrough emerged in 2002 when Caspi and colleagues find that individuals with a 2R or 3R version of the gene exhibited aggression, but only when they had difficult upbringings"
So the fact that the difference between black and white European is a mere 15% clearly shows it has less to do with genetics but socio economics status and upbringing, and this is without taking in account Single mother rate in AA's and the history black people have been through in the US, there are black people alive today that lived in the Jim Crow era.
Plus this study was conducted on a really small scale and wouldn't pass any peer review.
Sabo, Hu and Hamer (1998) found among 539 whites and 52 blacks that 59.1 percent of blacks had it and 33.1 percent of whites had it.
Beaver et al. (2013) found that 52.3 percent of blacks had it and 37 percent of whites had it among 2,574 whites and 164 blacks.
Rosenberg et al. (2006) found 54.1 percent of blacks and 36.1 percent of whites among 37 blacks and 274 whites.
37, 52 and 164 is nowhere near to be considered valid.