If truth about every person was known, I suspect millions fit the triad to some degree. And while a large percentage of killers and serial killers might fall into this group, I doubt all do. I suspect that the triad is no more than an indicator of a fairly significantly (2-4% maybe) group of people who are generally messed up. Maybe there is a large match to sociopathic tendencies but killers and serial killers is a tiny part of that group and probably not 100% exclusive to sociopaths.

While all squares are rectangles, not all rectangles are squares. Or if stretching to the triad you are probably talking polygons not even close to identifying squares.

McDonald's triad theory is just a sign of how hard is to apply scientific method to humans (no controlled experiments in the life and death arena). Also how little statistics mean when sample sizes are small and data relies on discovering inner facts in a world of secrets and skewed and lost memories of the past.