It is impossible for the police to be at the scene of every crime while it is happening.
Occasionally a criminal will linger, Law enforcement will conduct a sting or they simply happen upon an assault in progress but most of the time they are not there WHILE it is happening! It lacks reason to expect the police to protect everyone at every moment.
You know who is always at the scene of an assault… THE VICTIM!
We know that anyone can be a victim so if we, the potential victim, used some of the tools that law enforcement uses to defend we would essentially have a protector on the scene when any attempted assaulted occurs! That protector would be US… aided by that tool… a firearm!