So if I say that I killed someone because God told me too then this is a fact, even if I am lying?
Ok, I see your point. But there are plenty of examples of people being active for years in fora and such saying how they believe etc. and then commit a crime stating it was their belief making them do it. If you analyse the past behaviour and the reason to commit the crime they give, if they match then it is an as strong fact as you can get when dealing with humans.