Lol, it's citizens would kill each other if they didn't, the very concept of government is rooted in the first principle of a Monopoly on violence, the word "should" implies a set of rules above the enforcer they are compelled to listen to, governance itself requires the ability to restrict violence from others you can't be the first at that without violence itself, a "citizen" is just someone useful enough to let live if that equation changes they will use violence. This is fundamental.