Multiple failures in multiple areas, all the little things that added up to a huge murderous event.
One thing to keep in mind, and I have heard this in more than one forum, the "I hear guys here say they would evacuate" and "my gun is to protect me and mine", yes, true, for a civilian CCW'ing legally, that is very true. No civilian is obligated to respond with armed force, and unless I miss my guess, that area is a no gun zone, meaning not one civilian firearms owner would have been lawfully on that campus armed.
However, the SRO is/was a sworn law enforcement officer, on campus to stop events just like this, and according to the training I'm told was given to many agencies since Columbine, was supposed to engage the murderer at first opportunity. Since the officer was able to retire as soon as he was suspended, I can possibly assume that it was/is policy to assign staff to RSO who are on their last years as a cushy "slide out the door" job. I would assume that the failure of this officer to follow his basic responsibilities might lead other agencies to implement a different policy, perhaps rotating the assignment among more engaged officers.
In the meantime, the newly retired officer should probably move, someplace far away, because he will never live quietly in that town/area again for what he failed to do. I have no idea how many kids were killed while he stood outside.