Alright so I think Ridley made it a lot clearer what Jace's dramatic past moment was without giving us all the details. Spoilers and some theorizing below
spoilers:end of spoilers
I think it is obvious that Jace when he was younger was careless or perhaps even Drunk while driving and killed a pedestrian in a hit and run, but likely that his father and his connections got Jace out of going to jail but Lucius then sent him away to military school. At least that is what I am taking away from this, of course I could be wrong. But basically I think Jace sees being Batman as repentance instead of Bruce seeing it as a mission statement, that he needs to struggle to save lives of people as a hero to make up for the one he took.