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.
No. That is manslaughter or murder, and not justifiable at all. He was not trying to kill you. He was trying to save his own life and did not touch you, so it is not self defense.
Look at it the other way. You both run for the parachute and he kills you to get it. Still manslaughter or murder.
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.