I share some of your concerns, in a way, given that I am someone who preferred that the Joker's origin is never seen. He just shows up up and wreaks havoc, not giving any indication of why other than he enjoys it. I never thought of him as one of those characters that should be humanized or sympathized with in such a way.
However, I'm trying to be open minded and there is something to be said for seeing something new on screen. I agree that the Joker is definitely one of those purely villainous characters, not existing in some gray half-way space, but I don't think witnessing his transition will necessary taint the end-product. In fact, seeing it happening could be exiting. We could end up seeing a story of self-actualization, but in this version the character's truest existence is a villainous one instead of a heroic one. Maybe we'll see his absolute joy at becoming something he never knew he needed to be until he achieved it, at the expense of everything and everyone around him.