Clearly a man who does what the Joker does is completely mentally messed up. However, just like Harvey Dent, the idea that there was some catylist to his mental breakdown is hardly obsurd.
Sure, however there is simply no reason for this man's tragedy NOT be a physical change. It contradicts - again - over 70 years of history. Changing the Joker's catylist is just like changing Batmans, or Harvey Dents. I wouldn't want either to happen.
On a personal note: I am incredibly currious about Batman: The Musical and I am almost done with your Justice League script - I truly enjoy it