We have to be careful when we’re talking about crimes committed because of alleged demons, gods, fairies, etc. One thing we can’t ignore is the possibility of mental illness. Religious imagery is a common theme in psychosis. So when people say “the devil made me do it” they may be a religious true believer who convinced themselves (or used the excuse) that they were not under their own self-control, or they may be a person with schizophrenia or post-partum psychosis that was genuinely hearing a voice or having a hallucination that told them they needed to commit the crime for whatever delusional reason they came up with.
This is why I think it’s irresponsible to say that religion is to blame when crimes like this happen (as reported by the otherwise awesome Friendly Atheist, Hemant Mehta). The individual involved could have just as easily said that an alien told them to murder their child. Or some generic voice. Anything, really. So if they are found “not guilty by reason of insanity”, good. They probably are mentally ill and need the help they will get if they make this plea (because the majority of the time, these people are sent to hospitals where they belong).
“Guilty” in the law, as is my understanding, refers to your ability to tell the difference between right and wrong at the time of the crime. In the case of a person with a mental illness, it’s possible that their delusion was so convincing that it overpowered their sense of right and wrong. Sometimes the delusion involves doing something horrible to save their own life (so the voice says) so they legitimately believe that they must do this or they’ll die. So, technically, they are “not guilty” but it’s because they are insane, therefore they are sent to doctors to get the help they need. They aren’t typically just immediately let loose. So I’m not sure what the objection is here. Does anyone really think these people will do better in prison?
We can’t responsibly blame religion for (some of) these crimes. I think most events like these speak to the more relevant issue of the stigma of mental illness and the lack of recognition of warning signs before “odd behaviour” turns into “dismembering children”. People don’t “get” psychosis. People are also prone to brushing off odd behaviour and make the assumption that behaviour is always something people can control. Is that true? Can we always control our behaviour? The law says we can’t. And that’s why the plea “not guilty by reason of insanity” exists. These people don’t belong in prisons, they belong in hospitals to be rehabilitated so they won’t hurt anyone or themselves again. To ignore these aspects of law and society just because religion is involved is irresponsibly fundamentalist.