I'd have someone with some serious social skills.
Con/Seduce your way through security, or better yet, a pilot to get through security.
Not easy. I go through a lot of airports and those guys love to unquestioningly follow the bureaucracy. Best to have your way in figured out before you get there. Pilot is a great idea, but flight attendant is even better, I think.
pilots and flight attendants have to go through security just like we do, except they use a fast track lane. they get checked following a few attendants getting involved in smuggling operations.
even if you do get a gun onto the flight, you've still gotta wait the couple of hours in the 'plane with 20-120 witness to murder as you cant really bail out without decompressing and murdering the rest of the passengers at sub-orbital altitude. you'd also need some kind of space/pressure suit and a bloody good parachute to bail out.
if you dont bail, but stay on the plane till it lands, you've then got to deal with a small army of police waiting for you on the tarmac.
you cant really hijack the plane without getting shot down by military jets (ATC would pick you up as deviating from flight plans). even if you could somehow avoid them, where would you land it? and what would you do with all the other passengers?
which ever way you cut it, its just a bad idea. if you desperately need to kill someone who's going to be on a sub-orbital and you must use a gun to do it, then shoot him either on the way to the airport, or while he's distracted waiting for check-in/security. at least you can arrange a getaway vehicle and stand a bit better chance of getting away with it.
if it has to happen on the plane, get your unarmed combat adept to do it, or use poison or a poison gas or something and plan a pretty mighty get-away
