Technically, it would have to be a mental condition that completely removes the person from reality and places the person in alternate realities, lower self-control, and hallucinations.
But, the mental condition would technically not be what "lead to" since most people with (even undiagnosed) mental health conditions do not go around killing people.
As for stabbing during exorcisms, that may be the idea is that the body will come back to its normal state once the demon is removed. It is like an alternate state similar to when the girl in The Exorcist stabbed herself and did all sorts of violent things but her body was almost normal once the demon was removed.
A man claims he was going to exorcise his son:
http://www.huffingtonpost.com/2014/01/24/bryan-adams-exorcism-son_n_4658249.html?ir=Weird+News
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