An armed person won't stop a genocide, but an armed populace might.

Genocides are not committed solely by governments. An armed and divided populace is just as likely to commit a genocide as they are to stop one. Look at the Rwandan genocide. Look at the mass shootings we have here by white supremacists.

All it takes is an armed populace that stands by while “those people” (their neighbors) are killed by extremists (their other neighbors).