The Colorado murderer was a law abiding citizen until he started killing innocent people. The problem is mostly with seemingly law abiding, seemengly normal people who snap for one reason or another. Even the highly trained security forces make mistakes in shoot out with criminals and you expect regular citizens with no training even coming close to security forces' to be accurate and stop a nut case or a criminal?
Islamic State blows up tower tombs in Syria's Palmyra (Reuters)
from the article: The gun lobby and the freedoms of the quiet American monster