There is no truly positive outcome here. The game is quite dramatic in this regard. What is better? Sacrificing thousands for a guy who helps you save the world and will be able do that again in the future thanks to his powers or let out thousands of feral spawn that will kill even more people than Astarion probably ever could? Both choices are questionable.
Tav gives him a new start either way. He's free to choose who he wants to be in both outcomes. If he wants to be a monster then that's his own business. Even as a monster he can always decide he wants to live differently. Life is not clear cut like this.
