Well, s/he only now saw the direct consequences, and V did seem to regret killing those dragons, saying, if rember, to not praise him/her after all the ordeal with the dragon, fiends and Xykon. And we must remember, there is a lot prejudice agains 'evil'/'monster' races, which include Chromatic Dragons. Also, Tiamat is insane, but that is part of her mythology, and I think that lack of worship wouldn't depower her enough, in core(pre 4E)D&D, Faerun and Planescape she also seems to be a paragon of Chromatic Dragons and their kin, as long as they exist, she seems to resurrecting and keeping a lot of her power. Then again, this is OOTS-verse, so here it may weaken her enough for Banjo to take over 
and then everyone will praise the new and bestest god ever, because banjo is not picky with his followers, and gives happiness to everyone, still I hope V gets over this n.nU