Por que?
The weight of pressure needed to fire off a shot varies greatly between types of pistols. Some need over ten pounds of pressure and a long pull for the first shot ("double action") and then the following shots only need 4-5 pounds of trigger pressure and a much shorter pull.
If the cop had his finger inside the trigger guard while "covering" the suspect, he has violated every rule, and should hang. The finger should not go inside of the trigger guard until a moment before firing, and only after the decision has been made to shoot.
If the cop had his finger inside of the trigger guard, with a light single action type trigger (4-5 pounds of pressure to fire), and the suspect moves unexpectedly, it's easy to almost "flinch" and fire off the first shot. The next shots are almost a reflex action after the shock of the first "bang" (even though that "bang" was your own.)
This will be a huge part of the trial.