^ Exactly. Tighter background checks IS a form of gun control. If the people behind the guns are the dangerous ones, why let them get guns in the first place?
This link actually references another mass shooting, Virginia Tech. The shooter had been committed into a psychiatric hospital, but records were not found because the background check system is flawed. He walked into a gun shop and bought a semi-automatic rifle (AK-47) and a pistol.
Of those denials, many people appeal and win back their rights to guns using the Second Amendment as their defense.
Sure, mentally unstable people can use other weapons, but there is nothing else capable of the damage a gun can make. It would be very difficult to commit any mass murder with a knife. Guns are fast, and long range. Knives are not.