He knew the slide wouldn’t cycle from the beginning. He was prepared to manually recycle the slide after each shot. Knew his subsonic ammo and silencer would cause the gun to work this way. Pro job.
Wouldn’t a pro have used a revolver so as to avoid the cycle failure and not leave casings as evidence? And not impatiently shot right in front of a witness, and in camera view?
I’m guessing this was someone who lost a family member. He didn’t want the paper trail of buying a new revolver, so he took the time to practice with subsonic ammo in an already available semi-auto weapon, and learned how to tap-rack the next round.
As for his calm demeanor, he might have puked his guts out just two minutes later when the adrenaline wore off and the shock set in.
Fired once... hesitated... then racked the slide.
If he tried to double-tap, this would explain the hesitation before racking the slide and tapping...
I’ve fired low-power ammo before with a manual cycling of the action... I also practice dry-fire at home with snap caps.
Just my opinion...