How do I respond? Any argument with those kinds of people never ends up well. They have their opinions, and anything short of complete agreement with their position will be seen as wrong.
I had someone at work today bring up the Sandy Hook tragedy. They knew I was an owner and felt that it was the right time to let me know that I was partly responsible for the problem because I supported Civil (gun) Rights. What I did was ask if they remembered when the old guy drove his car through the market and killed a bunch of people. Obviously they were at least familiar with the incident and replied as such. My next question was what was done about it. They responded by talking about how restrictions were placed on elderly drivers, driving tests, and educating the public on talking about driving with their elderly relative. I asked if they supported those solutions, and of course they did. I then asked why they were not upset that they did not immediately ban cars because they were dangerous. Their reply was "that's ridiculous, the guy drove the car." I looked them in the eye and said "and the crazy kid pulled the trigger" and walked away.
Needless to say I felt a little better. The look on their face told me that they had never even considered that, but the seed of logic was now planted...