I doubt Vayne was going to annihilate the entire population of Rabanstre, devastate it yes but not complete destruction. His goal was war, to burn out the flame lit candle of strife with a kindled fire of his own melting the wax foundation (I know I played FFXII to much -_-).

And I have to argue Rabanastre did not surrender while its true the city itself submitted to Archadia's rule the resistance movement based within its walls did not. Hence the need to go and bring it to its knees- Being an imperial empire and all.

I think in the end for someone with so much responsibility of having to bear the weight of challenging the Gods to there self appointed rule he kept himself stable enough. He killed his brothers and possibly murdered his father but you can't say he did it out of spite or that he didn’t feel remorse for his actions. He was a man with alot on his hands and the fate of mankind rested solely in his palm if only his ambition was successful.