What kind of crossover are you hypothesizing? Shared demographics or individuals who do both?
Factors in common. Are there demographic, environmental, cultural, or other factors that are found in young men who go on shooting sprees or join violent political groups (left or right). I am sure that people who study recruitment into terrorist groups, for example, know this. Here in the US, for example, you probably don't have the religious or cultural aspects that inspire young men to plant an IED on a Middle Eastern street.