"shooting the messenger
No. If he'd just reported the problem, or hacked it while on the ground and stationary, then yes. But apparently he hacked into and changed the operation of an aircraft in flight.
How could be absolutely certain that this would not have any other consequences? It's not impossible that he could have crashed the system badly enough to crash the aircraft. He was utterly irresponsible and deserves the book thrown at him. An aircraft with passengers is not his toy to play with to show off what a great hacker he is.