I voted evil, but for slightly different reasons (that some touched on).
I will accept that killing all the black dragons in the world, either one at a time or in one fell swoop is not an evil act. The MM says they're evil, and that means they're OK to kill.
In theory, killing them one at a time, allows for a safety check of "is this one evil", but let's assume most parties don't do a detailed background check when they encounter a black dragon and it comes rushing at them, presumably not to hug them. So it's a wash.
What is evil is torturing them, not for information (which has it's own issues), but for the sole purpose of making them suffer. Whether V killed and re-animated the dragon, or held her, making the dragon watch as he killed her family (or species) was meant to hurt the dragon. Casting Familicide may qualify as a a pre-emptive strike. Making the dragon watch was torture for the sake of torture.
IMC, the D&D code of combat morality is as follows:
It is OK to kill something that is attacking you or somebody else (self-defense)
It is OK to kill something that attacked you or somebody else earlier (retribution)
It is OK to kill something that is actually planning to attack you or somebody else (pre-emtive strike)
It is OK to torture "bad guys" for immediately verifiable information (where's the bomb)
The first rule is pretty obvious. If you get jumped by orcs, you kill them. If you come across a caravan attacked by orcs, you kill them.
The second rule covers past attacks. You come across a village that was raided by orcs last week. You go hunt them down and kill them so they don't do it again.
The third rule covers an attack that is pending, but hasn't happened yet. You have information that shows the orcs are going to attack, so you beat them to it. This rule is also the fuzziest to apply. What if the information is suspect? Guilt is obvious in the first rule. Relatively proveable in the second. However, someone planning to do something is not the same kind of crime as actually doing it. There's still the chance they may change their mind, or even that the intent for the plan is misunderstood. What if the orc plan to attack the castle is simply a contingency plan and build up of forces due to increased tension and patrols by the king?
V's use of Familicide is a pre-emptive strike. Black Dragons are "always Evil" so there's some safety in the plan. However, he has no proof that more black dragons are going to continue the vendetta. So it's a grey area, and I'll call it a Neutral act, simply because there's good logic on both sides.
The forth rule, on the use of torture is there because some scenarios come up where a good party does need an answer from a bad guy. I'm not a pro-torture guy, but rather than tangling the game up in morality, I leave a valid scenario open, which is to torture to get a specific piece of information that is verifiable. "Who's working with you" isn't too verifiable, as the Salem Witch trials proved. "Where's the key to the door" is verifiable. If the bad guy knows it, he can tell it, and if he tells it, the party can get the key, proving the accuracy (and thus ending the torture). Like the third rule, there's some grey area if the bad guy doesn't talk. Is it because he doesn't know, or because he's resisting. This is where torturing quickly turns from a useful tool, to an Evil act. That's sort of the brinksmanship of torture. Imagine it as a set of skill checks with the DM vs. the party. They either get the information, and the party is not doing an evil act, or they fail. Since failure isn't "known" to the party, they either have to stop and accept the prisoner doesn't know (still remaining good), or risk another round of torture to try again. On a prisoner that knows the answer, it's a safe bet, they'll get the answer. However, if they push to far on a highly resistant prisoner, or they over-torture a prisoner that doesn't know the answer (didn't stop when they reasonably should have believed him), then the party fails and has done an evil act.
I just made up those general rules, but they describe how I generally look at things and rationalize what a "good" party is allowed to do in a typical D&D game. The trick is to make it OK to go around killing orcs for the most part, but point out scenarios where that isn't OK.