Violence decreases with less unwanted births, a higher general level of education, and the evenly-applied rule (and understanding) of law. Put your time and money to work on those fronts and you will see a material decrease in violence.
If you live in a place with high gun-ownership, you aren't going to effect change by trying to ban the guns. Advocating firearms safety education would be a good place to start, and believe it or not the NRA is probably your best hope for bringing a culture of firearms safety into your community.
There's plenty of room to argue about rights or the difficulty of actually reducing access to weapons. Saying that low hanging fruit is really gun safety, hotlines, preventative mental health etc. etc. etc.
BTW, I come form a country that probably has one of the highest rate of adults who can use & have access to an automatic rifle in the world. There aren't many incidents of this kind. I know that availability isn't everything.
Most killings are not conducted by the super resourceful, premeditated unstoppable type of people.
http://www.nytimes.com/imagepages/2007/04/21/weekinreview/20...
So if hackers should work on anything, maybe it would be some form of help for white males to get through their mid-life crisis.