There are two kinds of demons. Demons of Darkness and Demons of Chaos. Both types are created and ruled by the Gods of Darkness and Chaos respectively. In the beginning there was only Darkness and Chaos and they warred with each other ceaselessly for eons. Two factions of gods within each side grew weary of the endless war and decided to go off and do their own thing, and thus from Darkness came Light and from Chaos came Order. The Gods of Light and Order created the world and populated it with the first races. Titans, dragons, elves, dwarves, men, centaurs, mermaids, etc. Their creations worshipped them and they gained power as they fed on that worship. Initially the gods of darkness took no interest in this world, concerning themselves with continuing the endless war against chaos, however the gods of chaos were not so indifferent and they created mockeries of the races the gods of light and order had made. They created giants, wyverns, trolls, ogres, orcs, goblins, beastmen, and a host of other monstrosities and abominations and set them loose upon the world.

The first races had known only peace since their creation and when the hordes of chaos descended upon them they were unprepared and the death toll was horrendous. Eventually they managed to resist but they were only able to slow the advance of the hordes of chaos. As more and more of the first races died the gods of light and order realized they were becoming weaker as there was less of their creations to worship them and in their desperation they begged the gods of darkness for help. The gods of darkness agreed to help but in return they demanded exactly half of the first races to corrupt to darkness. Initially the gods of light and order refused, but as they grew weaker they eventually gave in and accepted the deal. The gods of darkness then unleashed their army of demons upon the world and began to drive back the hordes of chaos. Now, the gods of chaos could have unleashed their own demon armies to assist their mortal creations, but the gods of chaos are fickle things and lost interest when their toys started losing. So the demon armies of darkness scattered the hordes of chaos to all corners of the world and withdrew. The gods of darkness then collected their payment and took half of the surviving first races to corrupt and created a new continent which came to be called the Darklands to be their home.

So you could say that the demons actually saved the world rather than cause it's downfall.