I'm wondering the same thing. I'm also wondering if this "horrible thing" is something Batman, while not being the one who actually did it, is taking responsibility for doing. I imagine this War is going to cause both in lives and in other collateral damage and Bruce is the sort who would blame himself if his involvement with Riddler resulted in a lot of people getting hurt anyway. He is siding with the lesser of two evils and his motivation for doing so is likely because he wants to protect as many innocence people as possible.