Stanley took the object that Sheila had passed along to him before she had gone to fight the mayor.
It was the gun that she had brought over from outside, the one that originally had 3 bullets in it. Stanley had wasted the first shot testing out how shooting through the shield worked, and then the second one actually shooting through the shield the graze the mayor.
So, now there was a single bullet left on the gun and he propped it up, ready to shoot.
This was the only attack in either his or Sheila's arsenal that could possibly harm the mayor, so he had to make it count. He placed his finger on the switch and waited.
There were 4 locations in the mayor's body that he could really hit. Two arms and two legs. Anywhere else could turn fatal and they didn't want that.
They needed the mayor's help in escaping from this town after all.
