Because the only way for shit like this to not happen in your world of gun regulation, requires a total ban.
And a total ban gives whoever is in charge of the government a monopoly on force.
People like you will call Republicans nazis and fascist...then want to remove any force multiplier from civs and then pray they don't get elected. You're naive at best and ignorant at worse.
The only way to stop severely mentally ill people from obtaining guns is a total ban? Are you absolutely sure about that? There's no regulation out that that would mitigate the problem?
Because criminals don't follow the law... because they're fucking criminals....you cannot be this dumb... murder still happens you know? And a ton of laws are in place to make it illegal.