It's interesting that Voldemort is so attached to the idea of this restraining power Hermione has over Harry. Does Voldemort believe that this restraint weakens Harry significantly, by restricting him from making morally ambiguous choices? There's strong evidence that Harry shies away from evil solutions (Horcruxes are not a valid immortality method while they require human sacrifice), but he is certainly capable of morally neutral solutions. Hermione's emphasis on doing the Right Thing in all cases might strike Voldemort as a weakness he can then go on to exploit in his counterpart.