As his body would presumably go down with the plane and be sufficiently damaged to make the stab wound unnoticeable, what would it matter? who is going to accuse you? Anyway I don't fancy your chances - you have to stab him, get the parachute out of its wrappings, put it on - unless you're an experienced parachutist I don't think it you're going to survive anyway.
First, you must get the parachute away from the other person. When they try to take it back, you could kill them and argue that it was self-defense; them trying to take away your only chance for survival.