So the assassin is working for Silva, who wants someone killed in the opposite building already in a room full of his own henchmen??
Why such an elaborate way of doing it, why not just get the henchmen in the room to do it.
Well, a gunshot at close range would make those in the room the prime suspects, and instantly traceable back to Silva. And I guess the whole party would have been seen going up to the room together (CCTV), so whatever method they would have used to kill the guy themselves in the room they would still be the prime suspects.