Only to point out the obvious...thirty years ago, the vast number of pilots in a limited passenger world had some military background (US, Japan, Canada, France, UK, etc). Flight service boomed in the 80s and 90s, throughout the world.
Airlines are pretty much left to a very small group of folks to recruit from now. You need to pass the physical and have your certification. Beyond that...there’s not much of a mental exam or check-out.
In this case, we probably won’t find one single bit of evidence leading to some Muslim ideology...so we end up with some nutcase who they hired and simply didn’t review for any mental issues.
This would lead you to this prospective....how many more nutcases are there...out of 100,000 pilots around the civilized world? One, five, twenty? At some point, you start to question your life and the trust you might put in some airline to safely get you from point A to point B.
It may well be that this guy was just a fruitcup that went over the edge and decided to end it all and take 150 or so others with him. Pretty awful.