Let it begin

Experts who speculate that Dan fits neatly into a diagnosis of Anti-Social Personality Disorder point to genes. Other psychologists note the traumatic event he endured as a three-year old...

This disorder causes Dan to do one of the most unforgivable acts...
Murder.

However when he uses his influence as a famous youtuber to cause his fans to also murder, his roommate/friend Phil Lester finally does something .