Arizona has some of the weakest gun control laws in the nation. To gun owners, those laws — which allow them to carry guns in their cars, in restaurants and other public places — are their last defense against unexpected evil. Gun control activists say the state's laws put weapons into the hands of the mentally unstable, and demand that federal officials impose tougher background checks and reinstate a ban on assault weapons. Pima County Sheriff Clarence W. Dupnik said the state's gun laws contributed to the shooting. He singled out the law passed last year that allows those 21...