Colin Kaepernick says the 2015 shooting death of Mario Woods -- a black man who was shot multiple times by police officers -- is what compelled him to become a civil rights activist.
Colin told his story to Paper Magazine ... saying he was deeply affected by the incident, in which 26-year-old Woods was shot 21 times by 5 San Francisco PD officers during a Dec. 2, 2015 incident.
Cops confronted Woods -- who was holding a knife -- after receiving reports about a nearby stabbing.
Officers initially used non-lethal bean bag rounds on Woods while ordering him to drop the knife -- but then opened fire with live ammo when they claim he became aggressive and a threat.