Good question - and one that Jyn still didn't know the answer to. What could she possibly get out of disobeying orders? And why would she risk anything on a complete stranger?

"I don't know - ", she answered, completely honestly. "I just couldn't leave you out there to die."

Or leave the others to hasten his demise along, and probably in the most sadistic ways.

"I'm Jyn - Jyn Erso."