School Resource Officer at Florida school

CNN reported today that the school resource officer stood outside during the shooting and waited. What he was waiting for, exactly, I don’t know. As someone in law enforcement, I would assume backup. This officer has, not surprisingly, resigned. What are your thoughts on this? Do you think he should have waited for backup—IF that was what he was doing—or did he have a duty to protect and serve...to forgo personal safety and try to take the shooter down on his own? Should he face legal repercussions for not taking action?

*a little more info: He was armed with a gun and stood outside for about 4 mins. He was employed through Broward Co Sheriff’s Office and the sheriff has said, after reviewing the tape, there is no reason why he should have waited to go in.*