It's possible that Manning's actions would cost lives. It's also possible that Manning's actions would save lives, perhaps by aborting pending military actions.
Actual metrics are impossible to calculate, but we should acknowledge that both possibilities exist.
I also think it's useful to consider that you can betray a nation or government, but net a positive for the world as a whole. Many people believe that the US government has overused classification of documents which could prove embarrassing to those in positions of power. Perhaps people like Manning act as a natural counterweight, ensuring that those in power never believe that their actions will remain secret for eternity; someday, some loudmouth kid might spew your secrets to the world.