School Resource Officer could stop the killing?
Not in this case, as the killer boldly went past him to gain access to the building, and the guard did nothing for up to an hour while the killer was butchering the kids inside other than to cringe while calling more competent officers.
Hell, he made up the lame excuse he did not have a key to the classroom rather than to simply break down the door to save the poor kids from being slaughtered. His inaction cost many lives.
The lesson learned here, as well as at Parkland Florida, is a determined killer can continue to kill and we cannot depend upon armed security guards to stop them as some are just incompetent or plain cowards.