Sandy Hook shooter broke these laws (10 at quick count) in committing his murders, including the murder of his mother. What law are you suggesting that would have stopped him? Where would additional background check have stopped this tragedy? What punishment are you suggesting that is more severe than the murder charges?
Murder of his mother
Unlawful carrying of a loaded pistol with out a permit
Driving to school with gun.
Underage adult in Possession of a loaded gun-(3+ counts)
Bringing a gun into a gun free zone
Criminal Trespass (school)
Assault with a deadly weapon-Gun
Committing A Felony With a Firearm
Possession of a stolen gun (4+ counts )
That's just a quick inventory; I've heard he broke more than 30 laws. Would 35 have made this not happen? How many more laws that do not stop these tragedies to you need, to what end? I'm honestly curious.