I would like to understand why he would kill not only himself but also 149 other people. Why he would do research on it before hand. Tell his gf, he is feeling better, so she doesn't worry?! Why was there no good-bye letter? Why did he not really seek for help ever? He only went to multiple doctors, to get diagnosed, but not really treated. Did he not want to believe? In which episode was he in before deciding to kill himself and so many others?
One of the greatest problems with humans is that they must always search for "meaning" or "understanding" in events. If the epidemic can be explained by a vengeful God; or the earthquake due to another God's displeasure on human actions, then people can seek comfort in that the event happened* because* of some reason; instead of it just happening.
The media and yourself have said the reason why it occurred was because he was depressed, or suicidal or bipolar. People, over a long period of time have looked at a large group of people who have similarities in their mood and behaviour and given that set a label, be in depression, suicidal or bipolar. People will want to say the reason he did it was because he was bipolar; but there are around 186 million people with bipolar, surely others are pilots too. If it were singularly due to his bipolar, surely this would have happened numerous times?
So, why haven't they done it too? Why haven't other people with bipolar acted in a similar way. Because being given the label of bipolar does not inform another individual on what a person with bipolar is going to be thinking at every given moment onwards. No two people with bipolar experience it in the same way. Whilst they might share similarities in fluctuations in mood and experiences of other symptoms, the causes, timings, thought-content are not going to be the same. *People are different from one another. * Because this event might just be due to an individual acting in an unpredictable, unaccountable way. Do you have a reason for every single action you take, every single moment? Or do you just act sometimes; and when someone later asks you why, you might say "I don't know" or you have to think of a reason?
People can do things without having a reason. You can't speculate on the myriad of things you asked, because we're not his family, friends, colleagues, doctors... We're not him. The only person you can have these answers from is the pilot himself, who unfortunately like everyone else on the plane is dead.
My recommendation for you is this: instead of speculating and questioning how this event might be related to bipolar, think about the people in your life. There is most probably someone with bipolar or another mental health problem. Think about how the media is demonising people with mental health problems. Think about how people might feel less willing to talk to their doctor about the problems they face; or mention it to you. Then think about how this might affect someone, if they don't feel as though they can seek help, or have the support of their friends and family.
The plane has crashed. We can't reverse that. The only thing we can do is try to prevent something like this happening again. So instead of asking "why" or for what "reason" or speculate on mental states based on what you've heard in the media, ask this: what can I do to help? What can I do to support people in my life who might be feeling this way? How can I raise awareness for support organisations?
Help make a difference in someone else's life, opposed to participating in a sickening media parade and contributing to the problem.