I don't speak English very well so you may excuse me. I also didnt catch everythings from theirs conversations. But I think the first couple discussing about their meeting are selfish and deceivers people. The man behind them wants to kill them. His wife doesn't feel good and needs aspirine. He wants to give her but he finds the poison in his pocket instead. Panicked that she may see it; he tosses it on the ground. Where this loud boy takes it and put it in our cup, why? Probably because he is a kid. the plane leans to the right and with the additional weight of the pill; only our cup slides down to the innocent man who dies. Making people go crazy about all of that. The kid walking outside on the wing? Probably a bug mentioned on the Voice Air page. I don't know if it's what really happened but for me it's the solution that makes the more sense.
Cordially Mamadou La Baguette.