Most TV crime dramas are whodunits. Columbo famously reversed things by starting with the murder, and making it a "How'd he get him?" Mindhunter, in contrast to both those approaches, is all about motive. It's a "Whydunit."
Beautifully composed and tensely paced, Mindhunter is darkly addictive. The show deals with more than a few disturbing concepts and cases - those with a weaker constitution for such grim matters may find the stark details a little challenging to bear.