Khoroshiy chelovek

No spoilers here - it's revealed very early on that the killer is the local cop who's assisting the Moscow investigator. The grey, drizzly backdrop, corruption, toxic culture of masculine mentoring, and omnipresent alcohol are all brought clearly into play as the reasons for Ivan's devolution into a 50+ victim serial killer. For non-Russians it kind of plays to stereotypes about Russians being emotionally shut-down boozers, but since this is based on fact I'm not sure it's just a stereotype. Nikita Efremov does a really good job of making you both like Ivan AND want him to get caught because he's clearly a closet lunatic. Yulia Snigir is very good as the Moscow investigator and her troubled family background is realistic enough to make you buy the idea that a woman THIS beautiful could be a cop (and a minimally-supervised one at that). Nine episodes might seem long, but it's well worth it for the character and social portraits.