That has also been answered before. The courts have said over and over that there is no responsibility to do so and they cannot be held responsible.
To be completely explicit, the courts have said the police do not need to protect anyone from any crime in progress. Period. Even if the crime is occurring right in front of them, the police do not have to do anything.
For two of the more notorious cases that spell this out, Google Castle Rock v Gonzalez and Warren v District of Columbia.
The former case had Scalia specifically saying that the police do not have to come arrest someone subject to a restraining order at the time he was breaking the law, only that the person would be subject to arrest later. Even though he was killing children, the police were found to not be required to come arrest him.
Warren v. DC is almost worse.
Then WE need a “police force” to protect us! D@mn this crap pisses me off.