The machine is only loud when it is actively scanning a patient which it doesn’t seem like was happening in this case. Otherwise it’s relatively silent. Also the big button is (in my experience at multiple hospitals) always in a different room behind a box that you have to open. My point being this wasn’t some knee jerk reflex where he had the gun pulled out of his hands and he slapped the button. He physically had to leave the room and find the button to do this.
Probably not that hard if you don’t care if the patient survives.
¯\(ツ)/¯