But before you pour out that dragon's blood and trash the Urn, you may want to recall that you have two particularly devout people in your party; it's unlikely they'll just shrug off your destruction of the most sacred icon of their religion. They're willing to die for their beliefs - are you willing to kill for yours? Because that's what you're more than likely going to have to do before they let you defile the Urn. Oh, and then there's the Brother who guided you to the Urn's temple. You'll probably have to kill him, too. But surely it's worth the death of a few to save hundreds of thousands, right?
But that's all assuming that the Darkspawn somehow do gain possession of the Urn - a very big "if". The Urn could serve as a beacon of hope in a land that's facing a dire threat, and as any great leader knows, without hope, even the mightiest army is doomed to fall. It may even rally more people to your cause; you simply don't know what the long-reaching effects of leaving it in place might be.
This is what it's like playing Dragon Age. There is no one right answer, one way to see things, no "good" or "evil" path. As lead designer Mike Laidlaw put it, "You're called a Grey Warden for a reason." Every choice you make has an impact on your world, but that impact may be almost impossible to predict; you may not see the final ripples of your decision until the final chapter of the game. BioWare's David Silverman refers to it as "Choice 2.0, if you will," and it's quite a bit different from the "hit save/see what happens" scenarios we're used to.
If the demo from GamesCom is any indication, the situations you face in Dragon Age: Origins will be some of the most nuanced ever presented in a videogame. I'll have more insight - and details about my hands-on time with the game (holy crap, it's good) - for you soon. Oh, by the way, that dragon in the picture? He's hungry and you're tasty. Hope all that pontificating about your moral quandary didn't make you too tired to fight.