"Crazy pilots" is a new thing. We didn't have these problems before.
We can argue about the merits of strongly locked doors to the cockpit. Obviously, locked cockpit doors would have prevented the 9-11 hijackers from succeeding. Or, they would have found another way to use a jetliner as a missile.
The locking mechanism to the cockpit is really not the issue.
Clearly, people with mental problems are being permitted to fly planes. Or rather, people with issues are not being permitted to take medicine, since pilots can't take Ritalin, Celexa, etc.
Along with the spate of violent cop incidents, the question must be: what has changed?
We are getting a different result: crazy mass-murdering pilots. What has changed in the pilot-choosing system, or the pilot-vetting system, that we are getting these results?
The stories are terrible and frightening. Flying is scary for me on a good day.
There used to be three people in the cockpit. A Navigator would be in there with the pilot and copilot. That position has been eliminated in commercial flights as unnecessary and expensive.
Maybe it's time to rethink that.