He's crazy and a murderer. It's clear that he should not have had a gun... at all... not just in public. The implication of what you're arguing is that there is no way to keep guns out of the hands of someone like him without preventing people from carrying in public... or something similar. I don't know him well enough to comment on that.
Keeping guns out of the hands of dangerous people is very difficult. But it does appear to be a problem that we're faced with.
To adopt a child, we had to invite (and pay for) a social worker to evaluate myself and my wife (and to a lesser extent our kids), repeatedly, and intensely, from a psychological perspective. We were scrutinized both together and individually on every aspect of our lives, including sex life, pornography consumption, firearms, and the strength of our relationship with each other. This was done over the course of years. The social worker got to know us, and we got to know her. This was done because of the great responsibility we were undertaking by adopting a child into our family. But it is no less responsibility to raise a biological child. And it is no less responsibility to carry a gun in public. It is no less responsibility to be a member of the police force.