The man who runs down the aisle in the plane would be Charlie on his way to the toilets. The stewardess got suspcious when Charlie, who needed a fix, seemed nervous about something, that’s why Charlie runs and why the personel chase him.

Spoiler if you have not seen the season 2 episodes:
The stewardess who gives Jack a drink and is talking to the nervous Charlie is Cindy, the woman from the tail section survivors who disappeared in the jungle…

What I noticed was that preceeing the air craft turbulence there was a guy who ran right past Jack as he was talking to Mrs.(can’t remember her name) who’s husband ends up landing in a tree fastened into a seat and is seperated from her with the Tail survivors. The conversation is so focused I had, like Jack, not paid attention to the people running past yelling and all. Shortly as the runners (there are people chasing him too) move off screen a dull thud can be heard and then the plane shakes. The pilots shortly come on requesting seatbelts be buckled and then the whole plane falls apart. My impression was that this runner was directly responsible. …perhaps carrying and detonating a bomb …just a thought. But that’s the fun of the mystery – our imagination is far more powerful now enticed with this puzzle.