How long can a man hold the gates shut?
If his will and his nerve is there, he can hold them shut for all his life. But a man's life is short, and the carrion fly overhead all the time. One day, he will fall to the ground and not be able to press himself against the gates any longer. What happens then? The city will be taken by the heathens and used as they like. Better to destroy the city. Better to burn it to the ground. Better that it had never existed at all. Is the man who destroys everything that the heathens would desecrate committing an act of desecration himself? Or is it more purifying to rot?
Why grow crops for the enemy to eat? Why fill the court with virgins for the enemy to rape? Let it rot, let it all rot, until one day even the enemy finds that like all things they must fall and rot themselves. And then there will be nothing left except darkness.