Gotta back up the poisoned drink suggestion if you want to kill someone on a flight and avoid a small army of cops waiting for you when you land. It's one of the few suggestions so far that doesn't involve tons of witnesses, it's probably comparatively easy (Though getting that poison past the chem sniffers is still a problem), and if you're really good you don't even need to be on the aircraft.
All very good points. I think that sometimes it's forgotten that most weapons are obvious dangers, but sometimes less obvious dangers are more effective. Land mines are an example of this principle in action.
So when you want to kill someone on a plane full of people, you think... how could someone die on a plane? Well the plane could crash. This plan involves a hacker or a sophisticated bomb/device and a HALO parachute. This plan is harder than it sounds. Also, you are usually a prick for killing a bunch of probably random people.
Next one? Poison. You have to get poison on board and you have to make sure it gets in the right drink and goes to the right person. It also helps to be able to control the situation. Sometimes posing as a flight attendant, air marshal, or even just a random doctor that is taking a vacation can help you maintain control and can make certain aspects of this plan easier. Change disguises in the bathroom before exiting the plane with the passengers.
The fake hijacking can be a good one, but it works better with more than one person. you can also follow your target as he goes to the restroom, wait for him to come out, push him back in, and kill him. Overt plans like this almost always require exiting the aircraft before it lands if getting caught isn't part of your plan. Air marshals are actually really rare on flights. I don't know if that's different in 2072 of an alternate timeline, but I can tell you that this plan goes sour easily if there is an armed opposition.
Asphyxiation, severe allergic reactions, nerve strikes, overdoses... plenty of ways to kill without a plane full of people screaming bloody murder, but you have to know a lot about your target in order to pick the right cause of death for them. If they had documented suicidal tendencies when they were a little younger or a close relative just passed then you can take advantage of that and fake a suicide on board. Just an example.
I'm sure there are a ton of ways that I'm not thinking of right now, considering the ridiculous number of ways a person can die.
What if you board on a wheelchair, make sure you get the seat next to the guy, kill him discreetly, then wheel him off the plane in the wheelchair after discreetly switching a couple of pieces of clothing with him. Hat, coat, tie. Lol.
There is one ammo type that, in retrospect, seems custom-made for this situation. Frangible Rounds.
Flechette, gel, and stick-n-shock are probably relatively safe too.