I'm not usually a defender of police, but I think in this case it's not that simple. There were two possibilities, from the perspective of the police outside the classroom, where waiting would have made sense:
(1) He had already killed everyone in the classroom, in which case, keeping him contained until they were sure they could take him down was the best chance to prevent more deaths..
(2) He hadn't done so, since he was holding some of the kids hostage in the hopes of negotiating his exit, and a half-baked incursion into the classroom could have caused him to kill them.
One you have a nutjob loose in a school with an assault weapon, there are no good choices available.... just an attempt to find the least bad choice. I think there's a lot of displaced anger here. People are pissed off about our inability, as a nation, to address something that keeps happening here with alarming frequency (even as it almost never happens elsewhere), and so they're looking for easy scapegoats. The left and right each have a reason to focus anger on the police -- the left because of a broader hostility towards the police, and the right because if this can be framed as a tragedy caused by having insufficiently aggressive police, that can divert energy from the gun control cause.