In the US, it depends on what state laws you're in.
In Ohio there is no 'self defense' claim, and you would have leverage over the other person by having a knife, therefore equal force wasn't applied and you would automatically be arrested for 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.
No, that is assault. You attacked someone. Self-defense means fighting back when you are attacked. In your example, you are the aggressor. The person with the parachute would be the one engaging in self-defense, not you.