This RD article cites parallels between two murder cases with religious motives. It poses this interesting question:
By what criteria should we decide whether deviant beliefs and actions are an expression of religion or evidence of a medical problem?
It goes on to note:
From a critical perspective, it seems untenable to assume that the categories of “religion” and “insanity” are mutually exclusive. And yet legal claims often assume that these two categories are neatly separate. Partly because of this assumption, the “religion or insanity” question often amounts to a game of “heads I win, tails you lose,” in which whatever interests are more established get to make the rules as they go along.
When will everyone figure out that on a Venn diagram charting religion and insanity, the religion circle would be entirely inside the insanity circle?
Religion is necessarily a subset of insanity, certainly as regards one definition of insanity: "Doing the same thing over and over [such as prayer], expecting different results."
It's not just Christian delusion. John Krakauer's Under the Banner of Heaven, details a grisly murder carried by two brothers - the Laffertys - against their sister-in-law and her infant child. All done in the name of a personal revelation in the Mormon religion.
Not to get completely off topic here, but this seems to dovetail into a blog post of Loren's about The Particular Problem With Islam. When people act on the bad ideas of religious belief and consciously harm others, are they insane i.e. DSM defined insanity? If so, they are not responsible for their actions. However, if not insane, then we as a society are quick to condemn the actor, but refuse to criticize the motivational ideas that lead to the harm in the first place. We hear the typical, "Well, he's not really a Christian and that's not really Christianity," excuse. As though that somehow mitigates the bad ideas and is a palliative for those adhering to the same doctrine.
And, of course, as we have seen glaring examples played on cable TV and the Internet, give due criticism to the bad ideas, and you're a "racist" and a "bigot."