She wanted to win the battle, didn't she?
I'm not at all sure about that. When the order to rid Cardassia of its native scum gave, the battle was already lost.
But destroying all Cardassians would not have altered the outcome of the battle much anyway. It's not as if it would have taken that much effort to point a few of the starship guns down and slag the big cities; Jem'Hadar ground troops in turn could not have fought against the Alpha Axis starships anyway, so they could be safely assigned to hunt down survivors on the surface. So it's difficult to see how exterminating the locals would have weakened the fighting strength of the Dominion defenders appreciably.
They were winning the battle before the Cardassians turned on them.
And losing it after the Cardassians turned on them.
The declared strategic aim of the Dominion was to destroy Cardassia and Romulus. Since the latter was out of reach, it made good sense to complete at least the former job.