In any given instance they might or might not. But as a matter of probabilities the existence of an armed populace raises the ante enought to be a significant deterrent.
It is just like seat belts. In a particular accident they might or might not save your life. There are rare incidents where they are actually harmful. But when you look at a large number of accidents seat belts are very effective in preventing injury.
Why the general populace and the media can understand this principle when applied in some areas, but refuses to accept reality in other areas is a mystery to me.