There’s a big difference here, between being a good witness and being the victim in danger of serious bodily harm if not death. All witnesses in this case were safely ensconced in their nearby homes.
If Z had stayed in the car, he would have remained no more than a witness.
“Be a good witness” means to stay out of the action and be a witness. It does not mean engage, follow or do anything that causes involvement.
Z is the perfect example of what can happen to persons who try to play cop.