I think you are referring to the point when they first caught up with the suspect.....they had tried to intercept him so to speak at that time....it would have made no sense to leave him behind and drive way down the road....since they did not know where he might run to next....when they first caught up with him if they had wanted to arrest him that was when they would have got out of the truck and pointed their weapons at him and told him to get on the ground the way cops do and Greg had been one for 30 yrs. and surely had gone through that routine many times.
And another point Greg was on the phone with his back to the suspect ....hardly the position he would have been in if they were going to attempt a interception.
Also....as pointed out when Greg was making a statement to the police regarding what had transpired he was in a state of shock due to the trauma and somewhat confused....saying as he was trying to remember and tell the cops exactly how it all went down ....saying....it is all a blur it happened so quickly or woids to that effect.
Another thing to consider............in the legal world there is a old maxim............no criminal intent....no crime.
They certainly had no criminal intent and in fact were trying to do the right thing by helping protect the neighborhood.
As Greg said much later.....'no good deed goes unpunished'
Definition of Criminal Intent: Criminal intent is a necessary component of a “conventional” crime and involves a conscious decision on the part of one party to injure or deprive another.
thelawdictionary.org