Most definitely anyone in that situation needs to be wary of secondary targets. Remember the guy in Walmart who confronted the active shooter only to get smoke checked from behind by the dudes girlfriend. If there’s one, and he’s down and out of the fight, scan for secondary threatsVideo is a great reminder that I need to practice my draw skills more often.
After bad guy is dead all six(?) armed parishioners focused and advanced on the bad guy. If trained as a group would it have been more appropriate for some of them to scan the venue looking for other threats?