If a civilian brandished a firearm at a parked car before murdering the driver that civilian would almost certainly be prosecuted, no matter what the driver did.
Conversely, if the driver of a parked car survived an armed robbery attempt by striking the assailant with his/her vehicle, the driver would almost certainly not be charged.
In this instance, the undisputable facts are that a teenager was sitting in a parked car, and was shot to death by someone wearing civilian clothes. Everything else is a question of fact, which is what jury trials are supposed to resolve.
In such a trial, the burden of proof would certainly be on the prosecution to prove beyond a reasonable doubt that the shooter was also the aggressor. However, not bringing the case to trial at all means the district attorney is basically taking the shooter's word for what happened and letting the whole thing slide.