I doubt that Devoe would have left just his dead body and the gun. He would have made sure that there were witnesses to him entering the building and Barry's and Iris's apartment, witnesses hearing the gunshots (how else did the police arrive at the apartment so quickly) and possibly hiding one of the bullet casings under a couch or end table that Barry could overlook.
Wonder how the Thinker is going to plant gunshot residue on Barry to show he fired the gun that killed him (or his body).