When you say crime scene you refer a place where a crime happened. How to refer to a place where a crime is about to happen?
The police wanted to stop the guy before he reached the ______ with his gun.
You are right that you cannot really call it a "crime scene" before the crime takes place, especially if the early arrival of the police might prevent the crime happening at all, in which case it will never be a crime scene. You could therefore call it the "potential crime scene". This succinctly expresses the idea that the location may become a crime scene whilst leaving room for the possibility that it may not.