Sam heard the sounds of a fight in the alley and realized that things might not be as cut and dried as he'd first expected. He got out his gun, but he also made sure he had a stake. He'd been around the City long enough to have picked up a few things on the various types of vampires. And bumpy faced vampires meant wood stakes. At least, he was pretty sure that's what he'd seen pulling the two women into the alley.
Worse case scenario, Sam could always claim that he'd tried to shoot one of the vampires and missed and hit Fred. Or... something.
He burst into the alley, gun drawn, ready to defend himself and eyes sweeping the scene as he gauged his next move.