The serial killer and mass casualty incident people are overwhelmingly white men with some level of privilege. They're not your most disadvantaged people.
BIPOC people are much more likely to be subject to criminal sanction. White men who practise these behaviours with their intimate partners for years before they engage in serial killing or mass casualty incidents don't get called out, don't get considered to be violent, because they are protected by the system that targets other men, so their partners are at risk when those behaviours are missed.