You may be interested in the Korean movie, "Tae Guk Gi," which illustrates exactly what you're talking about and occurred during your parents' or grandparents' lives.
Err... the movie shows what happens when the government forces people to shoot the bad guys. When there are two governments (The Korean War), brothers end up shooting at each other.
To be sure, there were two governments in the US Civil War as well. I'd say it's a pretty rare situation where a unified government turns citizens against each other with guns. Maybe The Great Leap Forward and the like, but even that was maybe more prosecutorial.