Negative liberty doesn't work the same way for people who won't or can't take responsibility for their actions.
And there is a presumption of a civil society. Other than that there are laws to cover the few. In fact;he broke the law by using his gun to attack other humans.
The only real question here is ;was there a breakdown in the system when he was able to purchase the gun ? This questions links nicely with the OP about mental health.
But to say that millions of responsible gun owners should lose their right to protect themselves is absurd. As has already been pointed out here ;authoritarian systems are good at restricting legal gun ownership.