You didn't fail him. You didn't know any of this would happen.

[ If Cullen knew... It's not his place to know, certainly, but if he did-- Would he be able to understand why Anders despises the templars so much, why he is so frightened? Why all mages are? ]

And how could you have known that Justice would choose Anders? That isn't your fault either. Spirits are not foolish, Ser Howe; they know what will happen should they take a willing, breathing host.