Police protocol, nationwide as far as I know, since Columbine has been to engage the active shooter regardless of backup. The one and only goal is to neutralize the threat, to the point of moving past wounded and dying without rendering aid if the threat is still active.

Even if the officer is alone.

What's worse is that the preferred response, if possible, is four officers.

Doesn't matter if they were told not to engage. No one needs permission to do the right thing. There was only one possible wrong choice to be made here and they made it.