FN-2187 wasn’t the first Stormtrooper who proved unable to fire his weapon at unarmed civilians. Twelve years before the incident at Jakku, MX-5352 could not bring herself to fire upon a small girl trying valiantly to shield her even smaller sibling. It was not an act of defiance, but a failure born of weakness and her officers treated it as such. MX-3532 was sent to Reconditioning and she never made that mistake again. It set the precedence for the next such incident and the one after that and the one after that.
FN-2187 wasn’t the first to desert either. No one even knew about the first one. QR-2033 had simply collapsed with a pained cry and then laid as still as possible for three-and-a-half hours until everyone had gone. The First Order never bother to collect their dead so they never knew she was not among them.
Everyone knew about YB-5365 who had walked away from his guard post on a remote mining colony. It took the First Order less than 72 hours to track him down and his very public, very painful, and very prolonged execution became mandatory viewing for all cadets. So did the next one, and the one after that.
FN-2187 was, however, the first to free a prisoner, steal a TIE fighter, and evade capture only to return to base to assault a superior officer and blow the place up. No one knew what the First Order would do when they finally apprehended him but it was sure to be correspondingly epic.