Arguably, even the deaths of the occasional non-evil black dragons and half dragons is not enough to make this act evil. It seems to me that you've got to go with intentions, if you want to argue that this act is evil. Either that of you're committing yourself to a moral position that says... not very nice things, lets say... about every modern act of warfare or embargo to occur since at least world war 2. I can only pursue this line of reasoning so far under the code of conduct, and the analogy between enemy soldiers and inherently evil monsters isn't perfect, but the basic idea- that its not generally considered morally wrong to kill off even relatively large amounts of civilians (non-objectively evil creatures) in pursuit of the reduction of your side's casualties- remains the same.
Maybe we'd have a better world if people didn't think that way, but they do.
Since I cast the disintegrate spell, I am the one that's getting blamed for the entirety of my group's actions? And therefore, my family must be tortured?
C'mon now. The black dragon parent took this to another level, and V is now making sure that no other member of this family will take it upon themselves to exact revenge on V and his/her family.
Anyone who has ever shot a kobold brigand (or, heck a human brigand) in the back while they are running away from their botched highway robbery should appreciate that the morals in D&D are a bit different than in our world.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.