Would you or I engage.
That's the thing, I don't know how much of a hero I am. Or how willing I am to sacrifice my life for others, until I'm put to that test.
I think cops in these "terrorist" situations, if they choose not to engage, should be held to a reasonable standard like any other gun owner.
One attacker... regardless of the weapon (WMDs aside)... yes, I think any cop should be reasonably expected to engage to attempt to stop the threat.
Two dudes with knives... alright, sure... probably worth the risk.
Three automatic weapons? I dunno... we're getting to a point where a cop, even with the element of surprise, might just be throwing themselves to the fire.