A lot of folks (most?) don't understand mental illness and the toll it takes.
And I think this is because they don't understand themselves. A comprehension of human nature is in all of us but people don't bother to take the inner journey that unlocks it. It's very interesting, though, because it's an adventure every time you engage in it.
This guy, he didn't just go "hey, I have 20 minutes on my parking meter, I think I'll just snatch this plane and have a joy ride instead." He planned this for a while. He had to do his homework to figure out how to fire up that puppy and get it off the ground. That homework was probably a big part of what made it happen. Once he'd figured it out, he felt compelled to go through with his "plan," ... eventually. The explanations that people give for things they do are generally close to worthless. He said "I'm a broken guy," well, he said that talking to the ATC, it means close to nothing.