A little common sense would indicate that anyone would legitimately be in fear of their life if an armed gunman broke into a hospital like that threatening peoples lives and imitating a terrorist. Therefore, I would think it would have to be ruled as a "tragic accident," and be a justifiable shooting.
These "unannounced drills" have happened before and it always end up being a major embarrassment for those who planed such a stupid event. I believe the last one posted in this forum took place at a pharmacy about a year or so ago.
Fortunately no one was shot in either of these cases, but the potential is certainly out there.