Eight of the shooters in the last 14 mass shootings bought their guns legally despite having either a documented mental health issue or a criminal history.
And if guns were to be made completely illegal tomorrow, whats going to stop the next shooter from finding one? Cocaine is illegal, I'm willing to bet there is some within a block of most people reading this. Even more likely if you live in Edwardsville. Prohibitions don't really stop the behaviors they seek to, they just drive them underground, where it harder to regulate.
What is a "mental health issue" anyway. Someone who had been committed? Someone who was depressed at one point? Someone who likes to drink? Were they deemed by a judge of law to be unfit to purchase/own a fire arm or is it a 'Doctor' (of a questionable field of science) depriving them of liberty/property, without due process of law? We don't have a clue how much we don't know about mental health. "Mental Health" today is no better than George Washington's Doctors leaching and blood letting him.