Dilan didn't really have an excuse for annihilating entire races. Calling it a necessity or the act of a Saint was stupid because he murdered everyone and everything.
Using the Flame of the Beginning, everything was destroyed. Of course, he didn't annihilate any 'good' races, or races that were known for doing good deeds, or if they provided advancement in technology or something like that.
He annihilated races that were considered rotten by the standards of the Queen of Immortality's database. These races possessed vile traits that were given to them at birth. All members of those races possessed these vile traits that forced them to do horrible things seven of their personalities would have been nice if they weren't in possession of the vile traits.