None of our rights are absolute. The government does have the assigned duty of protecting the citizens. Any act to protect citizens will, in some way, limit the rights of citizens.
The question isn’t therefore whether it is proper for the federal government to have any restrictions at all on gun ownership. Just like with free speech rights, there are valid limitations based on the rights of each person to life and liberty.
I still don’t know if Cho was known to be mentally ill sufficient to trigger valid government action against his right to have a weapon. He certainly SEEMS to have been, but I wonder how “serious” this stuff really was at the time, compared to how it is judged now that people know the end result. It’s easy to look back and remember things more harshly.
I would note that nothing in the constitution explicitly states that a felon can be denied his right to free speech or to own a gun, but few would argue that convicts should have CCW privileges.
I understand what you are saying, but I tend to be a strict constructionist when interpreting the Constitution. The right to free speech, for example, is absolute. If absolute free speech doesn't work, then amend the constitution. I feel the same way about the Second Amendment. The right to bear arms is unqualified. If modern society needs to omit from that right people who have been convicted of certain crimes or certifiably crazy, then we should amend the Second Amendment to provide such limitation. Whether the issue is free speech or firearms, I don't trust legislative bodies or political judges to make these decisions.