Calm down with the terrorism stuff, this thread's going to go downhill.
I let people in unarmed helicopters and planes go because I fly around a lot, practicing landings and such. If they look like they want to kill me, I try to shoot them down. I never kill other players on sight, that way I can make friends and not be known as some trigger-happy douchebag who only bought this game to annihilate everything that moves. Of course, sometimes that's the point, and there is the time for that, but I want to be known as a nice person. I usually greet people by putting my gun away and doing the salute action. If they kill me, I try to go after them, unless I don't care at the moment. Sometimes they don't kill me and I meet friends.