to me, i think the idea of "vigilante justice" comes down to whether or not the action taken was in opposition to something being perpetuated by those who are supposed to meter out legal justice.
simple scenario: if instead of a cop it had been just some random person that gunned down a black kid in the street, would the name have been released?
or, if it had been a white kid that was shot, or any other combination of race and cop/citizen, would the name have been released?
history and common expected behaviors indicate that yes, the name of the shooter would have been released.
that in this case it wasn't, coupled with the wholly unacceptable behavior of the police force in general in the aftermath of the shooting, makes it clear that the formal channels of social justice were being subverted in an act of self-interested cronyism.
in that case, i'd say that "vigilante justice" is not only acceptable, but necessary in order to foster a free society - when you let the formal bodies insulate themselves and act above the law, you are perpetuating a fascist state.
it never ceases to blow my mind that the same people who will call a democratic president a dictator for trying to give everyone healthcare while freaking out about jack-booted thugs instituting liberal re-education camps will blithely accept police killing people and then the department trying to hush it up.