Maybe all those security briefings he skipped could have prevented this?
Sadly, no, I do not believe so.
Remember that he thinks America deserves to be punished, so if an attack happens, he will not be too exercised by it. This is most probably why he skips the meetings, along with basic indolence.
Maybe Hillary is trying to get rid of O in order to clear the field come 2016?
Not impossible: the Clinton's are never to be trusted, and are always to be interpreted as having the worst and most selfish intentions.
On the other hand, they are liberals: recall Billy Jeff's refusal to eliminate bin-Laden when he had the chance in the 1990's.