Yes, at its root the spell is evil, thus the act was evil, but it's a justifiable evil and, as a DM, while I would enforce a penalty for V's action, which is obvious from the deal that was made to gain such power, not all of it was evil. While I'm sure some may disagree, in and out of game, I've been of the opinion that sometimes bad things have to happen, in order to protect the greater good.
The dragon obviously had a blood feud, one that would have continued, so V, caught in parental rage, did what had to be done, sadly I'm sure that V's family is going to be, understandably, disconnect for some time, though.