Even if there was no dead whore in the room, photos of Geary with a whore 'just playing around' would be enough for the Corleones to get Geary. I think by killing the whore as well, the Corleones just wanted to make a point. A very strong point.
I don't see Geary as worst character though. I think the character was necessary, in terms that this was to show how corrupt the system, and I think it was well played by the actor. You can feel the evil and the almost sincere respect on Michael driven by humiliation and interest.