An unanswerable question

[Chad32]

One problem is that some mentally ill people think that God is talking to them when he's not and do terrible things.

Why do you assume that mental illness is the correct diagnosis in those cases? How do you know God didn't tell them to kill? What's your criteria for distinguishing a God sanctioned killing as opposed to one caused by mental illness?

There isn't a criteria, since we can't prove that gods exist, or that they talk to people. Until you can prove that someone who genuinely hears voices isn't just a schizophrenic, we're going to call it that and try to keep an eye on them.