If a person is mentally, and emotionally so far gone for whatever reason that they SERIOUSLY CONSIDER shooting people, and take the actions required to do just that, "Legal consequences" aren't even on the radar..
Some people are just evil. As Christians, we should know this better than anyone. If they're truly and dangerously mentally ill, then they need to be put in an institution away from society.
It's really a SPIRITUAL issue primarily (2 Chr 7:14). You can make laws till it comes out your ears, and it will accomplish nothing
Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment.
For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, for he is God's servant for your good.
But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer. Therefore one must be in subjection, not only to avoid God's wrath but also for the sake of conscience.
Romans 13:1-5 ESV
Eliminating guns would work (we'd have to stab people instead), but wouldn't be a practical possibility, since those who'd actually turn them in would be the ones who wouldn't use them illegally anyway..
Hence the need for the state to wield the sword.