Shots to the head are extremely difficult, and police training, for example, often emphasizes 3 shots to the chest as having a greater possibility of a successful immobilization of the target. Some of the descriptions of the incident in this thread also say that the victim turned to face the shooter after he was shot in the leg, and then was apparently shot frontally. More details will emerge, I think. Lack of a shot to the head does not mean a non-professional assassin.
One thing that is of interest is the pistol jamming. The shooter is knowledgeable enough to recover from the pistol jam and complete the murder. The unused rounds ejected and left on the sidewalk indicate some difficulty, and perhaps a lack of experience with the pistol he used. However, if the shooter was familiar with the combination of this particular pistol, silencer and ammunition, a jam shouldn't have occurred. Possibly, he hadn't tested the ammunition or the pistol. Another possibility is that the murder weapon was left for the shooter for retrieval somewhere, and that the weapon wasn't his.