I just started watching this, it's a three-part true crime documentary series based around the killings of Luka Magnotta.
I don't want to say too much in case people aren't familiar with it (I know it's true crime but maybe some want to go in blind).
Essentially it has interviews with online vigilantes who came together after a video of a man killing two cats appeared online in 2010. They formed a group to try and discover the man's identity but as they get closer to him it gets more and more disturbing.