Often, having a "climber" kill someone is the boss's way of testing his loyalty. If he does the hit on orders (usually without additional compensation), it's considered the last step before he's made. In "Donnie Brasco," Pistone was ordered to whack Bruno Indelicato, which would have been the last step before he was made. However, many mobs use contract killers for important hits because they don't want any of their own implicated--they could be squeezed by the cops and rat out the higher ups.