There were a lot of mistakes, first and foremost was the FBI really screwed up, big time! It just spirals downhill from there right to the armed coward that waited outside for 4 minutes and did nothing. Having said that all of this could have been avoided if the FBI did their job.
It was another avoidable tragedy. Prayers sent to all those impacted by this nightmare.
It definitely sounds like this nutcase slipped through the cracks at the FBI, but I believe the local authorities truly dropped the ball in this case. I read that they received 23 calls about this weirdo over the years and yet he was able to march right into the school and do what he did. Local police (including the school resource officer, from reports) knew what he was, the school administration knew he was, and the kids in the school knew what he was. This SOB's name should have never hit the news. With everything known about him prior, he should have been living a life of anonymity in a straitjacket somewhere.
In regards to the resource officer, I'll withhold judgment. We don't know the entire situation and I'd like to give him the benefit of the doubt. Not looking good for him, though, and he'll have to live with his choices for the rest of his life.
The internet is full of "if I were in that situation, I'd have..." type statements. That's all good and well, but until we *are in* that situation, we really don't know how we'd react. I'm sure even adrenaline and fear get the best of even the most well trained officers at times.