Why can't someone do both?
1. More than likely you are not trained to engage an active shooter in a gunfight (I don't think it is easy).
2. You have no duty to do so
3. You are not a LEO
4. You might get shot by some LEOs responding to the shooting
5. See number one above.
I can't imagine any jurist in his right mind convicting you of a crime or awarding a judgment against you for stopping a potential mass murderer.
I can definitely see it happening and it does all the time (mostly civil) to police who are sworn to protect us. Don't think for a minute some civilian wouldn't sue you for mental distress or damages if someone shot them by mistake trying to take out a BG.