The weapon that the shooter used does not look like a VP9. Watch the first video and notice the hand movements to rotate and unlock the bolt. Then look at the surveillance video as the shooter reaches over the top of the pistol to pull the slide back.
The shooter was obviously familiar with shooting a pistol but, IMHO, he probably didn't have the Nielsen device sufficiently lubricated to allow the suppressor to move forward from the gas pressure then slap back with the spring to unlock the barrel.
On the other hand he may have used a suppressor without the Nielsen device, knowing that he would have to manually cycle the slide.
Watch the next video in slow motion. Also recall that the article said that police found three empty cases and three live rounds at the scene.
When the shooter comes in view, he appears to be making sure the suppressor is screwed on tightly. Then he takes aim and fires the first shot. It appears that the slide does move back a little and gas escapes from the rear of the barrel and slide. He still has to reach over the top of the pistol and pull the slide back. That accounts for the first empty casing.
He then attempts to fire the 2nd round but notice that the pistol does not recoil. He pulls the slide back after pulling the trigger. That accounts for the first live round which was ejected.
Now that the next round is chambered he aims and fires again. The pistol recoils but with less smoke than the when the first round was fired. Regardless, we know that this is the 2nd round fired.
He pulls the slide back and ejects the 2nd empty casing. However, the slide does not return fully back into battery. He has to slap the back of the slide to push it in place with the left hand.
That doesn't appear to work so well as he looks like he's pulled the slide back again without firing that round. So the 2nd live round is ejected.
It appears that he still has some trouble getting the slide to move forward after ejecting the 2nd live round. As he's moving toward the victim, he moves the pistol in front of his body and out of view of the camera. He may have pulled the slide back again and ejected the 3rd live round.
He continues to move toward the victim and takes the 3rd shot because the pistol appears to recoil. Immediately after the 3rd shot is taken the shooter lowers the pistol sufficiently, but in front of his body out of camera view, and apparently pulls the slide back again ejecting the 3rd empty casing.
Now that the 4th live round is chambered he continues to walk over to the victim as if to fire a 4th fatal round but makes his exit instead.
Remember to slow the video down all the way and let me know if you agree.
IMHO, I don' t think this was a professional hit man. If he did this for hire, he wasn't very good at it. If he was a professional, he would have closed the distance to the victim to the point of almost sticking the pistol in his back and firing one round then making a hasty exit.
Speaking of the first round; also notice that his first shot hit the victim in the left hip/leg. At that distance, an accomplished pistolero would have been able to hit the victim in the back. This was possibly caused by nervousness on the part of a first time assassin.
If he did have a suppressor with a Nielsen device he probably didn't take very good care of his equipment. He might have been practicing with it prior to the murder but didn't clean the gun or the suppressor.
Like I said earlier, he appears to be ensuring that the suppressor was on tight before firing the first round. How many of us can attest to our cans wanting to come unscrewed after a few rounds?
After you have gained some experience you learn to prevent the problems of suppressors coming unscrewed. Someone with more experience would have ensured the tightness of the suppressor even before coming on the scene so it would not have to be rechecked, wasting time and possibly compromising the mission.
So he likely had some experience with using the suppressor but not enough to be 100% confident at this point. Some of us experienced with using suppressed pistols have either mitigated the problem of them coming unscrewed with blue Loctite, plumber's tape or in some rare cases, Vibratite.
Also note the fact that he experienced three misfires! Amateur reloader? Really bad ammunition? Poorly designed or poorly maintained pistol causing a light primer strike? Who knows but if the shooter was a professional hit man, he was the lowest bidder for the job.