Police or military. Whoever is doing the oppressing. It could also be their families at home or school. It makes them aware that joining an oppressive force is likely to lead to retaliation and makes them reconsider. When almost half the population is armed, as in the US, they'd have to always be paranoid about being shot by any random person. Or if they're mass murdering their citizens and you think you might be one of their victims, you can at least take some of them with you first.
The point is if you're e.g. the victim of a genocide or other mass killing, you don't just lie down and hope it stops. You recognise that you're in a war. Denial isn't going to help.
