Or their friends, or the friends of any of their various slaughtered family members, or anyone who just plain objects to people going around slaughtering entire clans of dragonkind.
Don't make this out as some kind of pre-emptive protective measure. Killing off an entire family line of dragons is far more likely to draw retribution than killing any single dragon - so V hasn't protected hir family, s/he's massively endangered them.
In fact, it wouldn't be uncharacteristic for a dragon-god like Tiamat to simply declare open warfare against all elves in retaliation to an act like that.
Except Tiamat knew (or should have known) this was coming vis a vis the oracle and did nothing to stop it's happening, like telling the ABD "um no darling this will set off all kinds of trouble"
Seriously if you killed Bobby in a "relatively" fair fight (or are we saying the slaying of the young adult was unjust ?) Then his mother humiliated you and was about to kill and then torture your family you would think just dispatching her would be the end of it ?
Maybe I'm paranoid but I would see a circle of escalating violence and threat best way to end that take out as much of the threat as quickly as possible.
"They pull a knife, you pull a gun. He sends one of yours to the hospital, you send one of his to the morgue."