Americans have this thing called the Constitution which states citizens have the right to bear arms. Now, when you allow government to make subjective decisions about who can own and who can't, it's no longer a right and nobody can have guns but criminals somehow always find a gun. If you know anything about what happened, you will find the shooter is actually a very normal person and not some gun nut so any amount of psych testing wouldn't have found anything. What happened was wrong and both sides could have done better to avoid such a situation. Doesn't really matter because shooter is going to jail. And many state do background checks when selling firearms so it's not like a felon can just walk in and buy a gun. But when you're not a felon, how do you predict a person's future behavior by some magic questionnaire?