Taking away the rights of an average citizen to obtain and use a gun doesn't prevent criminals and psychopaths from getting guns. It just means that regular people won't be able to defend themselves.
In the United States, illegal guns are used in almost 90% of the gun-related crimes. With Mexican drug cartels such as the Los Zetas, Barrio Azteca and Los Mazatlecos expanding their operations to the Southern United States, it is only a matter of time before states such as Texas going to get swamped from illegal firearms. And if you take the guns away from the American citizens in these states, it will make them sitting ducks against the drug peddlers.