I agree with you for the most part. Police often can't even be bothered to come out for noise or parking complaints. But I can hardly imagine a situation where the police wouldn't respond to a domestic violence/threat of domestic violence complaint.

And yes, once they're there, how they respond to it does involve a lot of discretion. I'll agree with that. But the man with the knife had just as much discretion as the officers did, and seemingly chose not to use it. I know that neither you nor I were there, but from the article, the officers even tried using the taser first, and the suspect still chose to be shot rather than to put the knife down and be arrested.