Psychology and Responsibility
My field is psychology. Whilst it would be absurd to assert that the pilot's possible family issues must have a bearing here, it is equally absurd to suggest that the possibility has been 'eliminated' by talking to his maid or his friends. Indeed, the person who would open up to their friends and let the mask drop in front of the maid is the kind of person more likely to 'cope' successfully. On the other hand, someone determined to keep up a front, ashamed to seek help, or in deep personal denial of the situation until the moment their family actually move out is more likely to be prone to 'snap'. Just to reinforce: I'm not saying that the pilot acted in such a manner, just that the reasoning for elimination of this possibility presented in some previous posts is unconvincing.
As regards an entirely different hypothesis, terrorism, multiple posts have expressed incredulity that responsibility would not have been claimed by now. Several answering posts have rationalised why the hypothetical terrorists might have chosen to remain silent. My personal hunch is that probably dozens of conflicting claims of responsibility have been made, that these are being looked into according to their apparent credibility, and that if any one of these claims starts shaping up evidentially as possibly genuine, we will likely then hear of it. Incidentally, this may offer one amongst many reasons not to put all known facts in the public domain: a good challenge to a claim of responsibility is to say 'OK, if you did it, tell us something about the events which we know or can verify but which is not yet public knowledge'.