Imagine a world where normal people don't die when they're killed.
Rather, they ressurect in a matter of seconds back to perfect health.
A world where there are no doctors because when somebody gets sick or injured, you can just shoot them in the face and they'll come back almost instantly all fine and dandy. The only way you can die is by plain old age.
But some people in this world are not normal. They have the "sickness" of being able to die from body damage or bacteria or virus. And such "sickness" can be transmitted by exposed contact.
In this world, such people are considered "animals" to be rounded up and killed. But of course, they want to live as hard as anybody else.
That's the main theme of
Shinazu no RyoukenOh, and also the main female protagonist is a meganekko packing as much firepower as your old-school FPS character. Remember, it's not evil to shoot at civilians in your way when they'll just regenerate next panel!