Torture, as well as being in itself an evil thing, helps to train the enemy. I don’t mean that the enemy learns how to prepare for particular torture techniques (though that will happen), but that it gives the enemy precisely the right incentive to fight more cleverly.

Because, if any enemy wants to avoid the discovery of information through torture, then they have the precise incentive to prepare all plans so that they remain as secret as possible, as well as compartmentalize all information, so that the effect of discovered information is limited. (Just as the French resistance did when fighting the Gestapo.)

Additionally, if the enemy counts their own lives as worth relatively little (and I expect you can think of examples of this kind of enemy in the current world) then they will be encouraged to give up their lives yet more readily than they already do.

Look at those two effects of choosing to torture: enemies will plan more carefully, and give up their lives more readily.  Yes, torture is both evil and stupid.