It wasn't a matter of the boy proving that he could kill. Harth had already seen enough to know that the young human possessed everything necessary to be a vicious murderer. What interested the vampire, rather, was to see how devoted he was to his addiction.
In forcing the boy to wait, to actually follow instruction, Harth was observing how committed he was to chaos. Most human murderers were impulsive. Weak in their desire. But, if the boy proved to be willing to walk through the fire to cause pain, well...that was certainly an admirable trait.
After a short walk to the cemetery gates, Harth pushed open the jagged cast iron entrance and motioned for his companion to enter.
"We've reached his final resting place. What you choose to do with him from here, well...that's up to you. As for me? Just consider this a spectator sport and I'm your very eager audience."