You can be depressive or mentally ill without wanting to harm yourself or anyone else. Let's not blame the murder of 149 people on depression. The question really is, how to protect against malicious pilots?
Improving the conditions for the pilots and crew is a good goal though.
Somebody has to be responsible. If the pilots shouldn't be trusted, why should those that would override them be?
Nope. You don't have to have any single person trusted - that's the whole point of having two pilots. This is just extending that basic systems architecture notion (having no SPOF) where it's lacking.
'I trust the pilot' does not imply that I think that the pilot will have a long and healthy life. There are two pilots because one might have a medical emergency, emphatically not because one might go mad.
In fact, if one does go mad, even with the other one present, there are plenty of ways in which the 'mad' one can run the plane into the ground if they so choose to.
So the 'two pilots in the cockpit' kind of redundancy is to protect against the cases of individual pilots becoming incapacitated.
Of course having another pilot in the cockpit will probably somewhat reduce the chances of a pilot with a mental issue being able to inflict damage simply because he'd have to either get physical with the other pilot or at a minimum would have to look them in the eye. But that's nowhere near what is required to stop a plane from crashing if one of the pilots decides to do it.
Which is one reason why it doesn't really matter if a flight attendant is present in the cockpit instead of another pilot. That's mostly a psychological issue, not a practical one.
It was not unusual for the Flight Engineer to be senior to the drivers.
But the airline accountants highlighted the personnel cost of Flight Engineers and eliminated them, without ever considering the operational cost savings they provided. Even in mundane matters like resolving departure issues without having to wait for the on-call engineer.
"How long to fix it, eng?" became "What's wrong with it now? Back to the gate."
Modern ships are filled with fancy electronics, and it's usually poorly designed with broken software, that few know fully how to operate and nobody knows how to fix. This problem is only going to increase as they rely more and more on automation, and it's not going to stop until there is an accident severe enough to prompt aviation-style regulation and safety methodology.