I don't think it's necessarily defending them to state that their behavior seems like the behavior of a mentally ill person. Somebody who was wandering the streets barefoot with an infected gash on their foot, mumbling to themselves and stinking to high heaven sounds like someone that has a mental illness to me. Someone saying they're Jesus Christ and being convinced the FBI and the KKK are trying to kill them sounds like a mentally ill person.
Stating that crazy behavior is crazy isn't saying it's acceptable. Miller being cast was clearly a disaster and they're committing crimes so obviously it's bad. Its just acknowledging that some kind of mental issue is at play here.