Did the authors of the Westminster Confession of Faith ever explain why they thought God could not be the author of sin? Ch3 of the W.C.of F. says "neither is God the author of sin".  

One can make something which is entirely different from oneself. e.g. if someone makes a cake they do not become a cake themselves.
  If God had a holy motive why Jesus alone should fulfil the Law {total obedience} then God might have a holy motive to withhold from man the grace to obey His Law/command.
   I can find no place where the writers of the W.C.of F. tried to justify the statement "neither is God the author of sin". Did they ever try to justify it?
The Free Prebyterian online copy of the Westminster Confession quotes three verses in support of the statement :

neither is God the author of sin,b

Free Presbyterian Church Documents
The first two are from James :

Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man:
James 1:13
Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.
James 1:17

The third is from John :

This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
I John 1:5

I was not certain but @curiousdanni notes below that these texts are original to the Westminster Confession and are not added by the Free Presbyterian website.