My theory?

Well.. Lets get this straight. He is Crazy, but he's not dumb. He's actualy very smart. Why do i think this? Well has he been caught by the cops or FBI yet? No! He hasn't.... This leads me to my next fact that he's Very well trained in assassinations and escapes... Almost like an Assassin. Could it be somone sent from another country? Or just an Anti-American Government Group?

The way we can catch him.... Hmmm.... Did you know that each gun has its own uniqe chamber marking? This Meaning that the shells left behind at the seen(which is shocking to me b/c a trained person would not do that, in fear of being traced..) Has its own uniqe chamber Mark. They can take this chamber mark and match it up with a certin modle of gun. After they have the information about the gun, they can contact every gun dealer in the area and see who they sold that gun to via serial number.

And i bet your thinking.."What if he sawed or filed the serial number off the gun?"
Well soke it in an electirck bath and Bing! The serial rises to the surface again.....