https://www.nytimes.com/2005/06/28/politics/justices-rule-police-do-not-have-a-constitutional-duty-to-protect.html
I really think an argument can be made that the Castle Rock case doesn’t apply here. In that case, the police were not on-scene. They just failed to respond.
Suppose you called for an ambulance because Great Uncle Gus was complaining of chest pains. But the dispatcher didn’t think the problem sounded serious. So no ambulance. And Great Uncle Gus died. Could you sue the ambulance company? Maybe not.
But now suppose the ambulance happened to be parked right next to Great Uncle Gus, and they just stood there and watched him die. Could they be sued then? I sure hope so.