i think the joker dosent give an explanation about his scars and wears makeup for the same reason batman wears a mask. thats really the only reason he baited him in to taking it off, he realizes like himself, thats something he cannot reveal. he's trying to show batman that every hero has the ability to become a villian once broken down to the ground, much like the devil was once a fallen angel. quite possibly the thing that made them what they are today is exactly why they are what they are. the joker specifically made two face what he is, to continue the game. at the same time, batman even created two face, because he could have doused out the flames on his face like 10 seconds earlier than he did. i thin that joker realizes this actual vengefulness in batman and might have been why batman left the coin.. as far at the joker, its much like killing the children or inocent people, he creates more villians to make batman crack. the whole game seems to me to be much liek the psychology of playing cards, sometimes one might have better cards but you manipulate the player into being weaker underneath better odds.
the joker card being the wild card, much like 'a dog chasing cars'
it just 'does'
but seems to have an overpowering ability over everything else that plays within structure