They're already spying on everyone and passing "precrime" laws. Passing ex post facto bills of attainder is not going to solve the problem by itself either. But to your point the above mentioned countries don't lack mental health care programs like America. Plus they have much more limited access to firearms. I don't think taking away people's rights every time some whack job exploits our system is a good idea. That's a sure way to end up in a dictatorship, especially when you consider the department of Justice lets lunatics walk the streets every day. They have all the leverage on a scared population in that regard.
We literally have Americans begging the government to take rights away, and you agree with that? Next step is your rights. You can bet on that.
How many school shooters had been undergoing psychiatric care? And also double that up with recent red flag laws. I don't advocate for sending my children to a school that resembles a prison. I do feel like there's plenty of laws written already that are not being enforced. Stop worrying about the millions of Americans who are not the problem and start treating people with a mental illnesses, while shelving their right to own, possess, use or purchase a firearm. It wasn't too long ago that 900 million was allocated to NYC for mental health, not one penny of that money is accounted for, but the mayor and his wife are in the Hamptons of you'd like to ask them.