My hypothesis is that most mass-shooters / mass bombers including terroristic ones tend to have not just low Serotonin levels, but extremely low Serotonin.
Low Serotonin is known to be much more common in violent offenders, and it's ALSO more readily found in people with morbid thoughts, or OCD patterns, it causes also instability.
Morbid thoughts, violence, OCD, and also instability? ALL SOUND like MASS-Shooters / Mass-bombers (Terroristic peoples)
Note that this map puts Poland & South Africa (Blacks) at the highest Serotonin level uptake & East Asians, Turks & Indians the lowest Serotonin level uptake.
Now Britain isn't terribly low in this map of Serotonin & BUT, when you include Histamine (Rosacea) + Red hair genes far more common in the British Isle bringing down Serotonin levels, then it's probably a big difference.
The depression map: genes, culture, serotonin, and a side of pathogens
BUT, if we speculate that Poland & Blacks have the highest Serotonin concentrations, and Brits including both Irish & Ulster Irish , East Asians & presumably judging by Turks who are heavily Arab related in genes despite not fully, that PRESUMABLY the reason why Brits (Irish & Ulster Irish included), East Asians,and Middle-Easterners commit more mass-shootings / mass-bombings & genera terrorism is because of LOWER SEROTONIN CONCENTRATIONS.
As to why Blacks have more "Thug" types of murder it's presumably because of more genes for Low-MAO-A.