How many pilots suffer with clinical depression? Many. How many entertain homicidal thoughts? Very few. Few pilots are going through "exactly" what this guy was going through.
How about suicidal thoughts? If you feel nothing, you feel no empathy, no sympathy, you don't care about anyone or anything. I can totally see his attitude being "I don't want to live, so I'm just going to crash this plane into the mountain. What are they going to do, arrest me?". Not necessarily wanting to murder people, but just an attitude of "I could care less". It's a thin line but luckily most are more morally sound then to do such a thing.
But to entertain what you said, ok, sure, let's say very few are going through EXACTLY what this guy went through. All it takes is one. Pilots need better access to drugs and help, knowing full well that if they leave to better their mental state, they'll be welcomed back with open arms so long as they aren't medicated for the trip(though I'd argue flying during deep depression is more dangerous and distracting than flying on Xanax or similar in a mutli crew environment). There's this huge stigma against drugs, but let's say Jerry the pilot is going thru a bad divorce, can't pay the bills, and is on the edge of giving up on life. He wants to feel better so he puffs down a fat blunt of weed 2 days before the trip to unwind and clear his head a bit, he then feels much better. Is he high right now? No? Ok cool, I'll strap right in to the back of his jet and not think twice about it. Let's say he's afraid to talk about it for fears of his current psych state being "outted", he can't sleep on the over-nights, and he's having suicidal thoughts(not even about crashing the aircraft) as he stares out the window in cruise since he's done nothing to ease his mind be it talking to people, self medicating, or getting help. Would I get in that plane knowing all that? Nope nope nope AND nope.
It's the 21st Century, we need to re-think some things.
I'm talking about your sister being normal and such one day, and the next she's in a hospital so she doesn't off herself? As I said, I've never suffered from this or known anyone well that did... but how common can it be?
I've had 2 friends kill themselves, one out of the blue, the other was always struggling but she seemed to be doing just fine and we had all just stopped worrying about her more or less. I've also had 5 or 6 go to the hospital for trying to do it after having shown no signs what so ever leaving me completely stunned. And I mean out of the blue, like I would bet million to one odds this person wouldn't kill themselves. And they actually tried, they weren't playing around. I know of many more who can be totally fine, social, and normal for a while then turn off their phone for 2 or 3 days, not even talking to their significant other, freak everyone out all to find out that they were just in a funk and didn't want to talk to anyone or leave their house. It affects a lot more people than you'd think. One day the world is your oyster, the next day you only leave the bed to eat, flake on all your plans and obligations like calling in sick to work for no reason, and keep the phone on airplane mode all day. It's real. And if affects a lot of people. A lot. Of the people I've mentioned, I'm one of maybe a small handful of people who has ANY idea they tried to kill themselves, no one else would ever guess. People open up to me, and man, is it real. I only know so many of the billions out there. Probability suggests there must be many, many sick people.