How serious was the break in protocol that he was fired rather than disciplined? I'm guessing his mistake was engaging the suspect before back up arrived.This should not go to the grand jury because it was a good shoot. However, he was fired because he was new to the force and failed to follow that departments procedures about officer safety. I agree with the firing, because of you can't follow the rules of your place of employment then you need to be let go. Especially when your conduct can directly affect the safety of those around you.
His termination from employment and the grand jury are completely separate issues. Most shooting fatalities, whether officer involved or not, are presented to a grand jury. Most, not all.
This is usually done to protect the integrity of the ultimate decision on prosecution, since the 12 members of the grand jury have the power to fully investigate all details, not just the portion that was released through the media.