Your general point is a valid one, but something lurks below in my opinion.
What about 'sin' prior to the churches' declaration of such a concept. Susan's extreme point is that the serial murderer, who plots ways to kill hurt and maim, is just making a "mistake" which can't be sin. Rather simplistic, don't you think?
So, let's back up a little. Your take sounds typically Marxist, basically being that idea that churches only exist, or exist mainly to control the masses. The 'sin' issue as you describe it, would be but one of several variants on this general Marxist idea. Not saying that are a Marxist, or that that is even bad, but is that what you are really saying? That no such thing as 'sin' exists other than it being an invention of religion? Too simplistic for me.