[INVAR]

Well, in that part of the world, the left hand is what you clean yourself with. There is a reason that “right” is good, and left is bad.

Wrong application in the biblical context. Psalm 26:10 tells us that bad things, evil things are also done with the Right Hand.

Right hand is the symbol of strength and action in scripture. It's what one does based on his will and devotion. Many times in scripture the Right Hand is used to describe what the forehead thinks or wills to do.

In the context you suggest - the scripture that tells us that our righteousness is as filthy rags before The Father, would also be served as what the 'left hand' does in terms of cleaning oneself.

[MHGinTN]

Might it have anything to do with which side of the bilateral brain controls which side of the body and is associated with which kind of reasoning?