The officers at that point had no reasonable expectation of being shot and then she came in with a shotgun leveled at them. Standing there aiming it at police officers will almost always draw a response ~ even in your own home.
The point is the homeowners probably thought someone was trying to rob them. We dont know if she knew it was the cops and she probably had very little time to make the realization before they killed her.
The homeowners were doing nothing wrong.