All Beliefs are Welcome Here!
My own personal answer would be that it depends on who you are and your own values. I do not believe in karma in the sense many people do. I think of karma as something you set up for yourself as a teaching tool when you are disincarnate. This being said I will further state my own personal views.
I have no qualms and an heirloom gun to back everything else up. My families well being is more important to me than someone who makes a bad decision and opens themselves up to cause and effect.
When I think of doing something on a massive scale I ask myself how would I go on empowering the spell so that it maintains itself. I would root the spell in a physical object and find something to empower it. I would probably use peoples negative emotions. To make the place better by removing some of the anger, hate and hopefully fear. The best example I could give is like this. I know some energy vampires who will draw in negative emotion and transmute it before it is drawn into themselves by creating a filter. When this is done you can actually feel the tension being drawn out of the room.
In my nice suburban neighborhood there was a string of brutally violent attacks two summers ago. One of our neighbors was almost beaten to death. I and a few of my coven mates did a spell to make them "glow" to police. We deigned that the people responsible for the attacks would be highly visible to law enforcement. The next day, they were caught (I'd like to believe I had something to do with that, but we'll never know).
As far as preemptively warding a neighborhood, I'd find that to be iffy. For instance, my neighborhood is heavily Catholic and I'm pretty sure that doing any Wiccan spell on behalf of those people would not be well accepted by them if they knew. So I wouldn't do it.
The exception for me would be in times of great strife, such as war. I would use any means at my disposal to prevent immanent atrocities that can be reasonably predicted to occur. I feel it's my duty as a witch to protect people's human rights in times of crisis.
Magic is a tool. And I'll apply it in any situation where I think it will be effective.
I'm a martial artist. I have NOOOOOO problem laying someone out flat if the situation calls for it, so why not do the same with magic?