I think also that black and Hispanic males are also more often from lower socio-economic strata, without parents able to shield them the way this kid’s mother shielded him.
Thus they are likely to enter prison system before they reach their going off peak, if you will.
Another interesting angle. Expanding on that a bit: This study seems to focus on mass-slayings. However, what about murders in all-race communities? IE: Detroit & Chicago? Seems the murder rate is way up, but not in a huge number of people killed at one time. However, it comes in smaller numbers as the gang-bangers take out their perceived rivals or react to being dissed.
seems the study is pretty narrowly focused...