This is incredibly fascinating.
For those of you who continue to say "not evil," may I ask:
If a paladin in your game performed the act of slaying the dragon and all of its kin to the Nth generation, said paladin would not fall? Is that right?
"Not fall" is the logical result if you believe the act is "not evil," but I just want to make sure.
In my games, gods and characters do not follow our modern values, but fantasy/medieval ones. The paladin would not fall for killing black dragons, just as she doesn't fall for killing evil people.
Also, in my games, revenge is usually a common motivation, and honor and customs being as they are, you have to expect that if you kill someone, their relatives will take revenge. The son that does not want to avenge his father is the exception, not the norm.
And finally, in my game, my NPCs - especially my evil ones - are not suicidal. After such a spell, just about everyone would think twice about messing with such a power. Some fanatic priest of Tiamat might - might, if Tiamat did not simply write this off as the ABD making a fatal mistake - want to take revenge, but almost all dragons, for example, would simply write a memo to themselves "Do not mess with that elf, ever!".
To borrow some old image: If I don't expect level 1 PCs to go after the ancient red dragon they hear about, I don't have dragons going after the "epic dragonbane mage" they hear about. Not unless those dragons have some very low intelligence and wisdom.