Why doesn't batman kill the joker?

i am not talking about killing scarecrow or penguin, just that bastard...i know the creators keep him alive for the story but besides that why doesn't he kill? ignorant people think he would be going on Jokers level if he did but he wouldnt the difference is joker is killing for fun and innocent people while batman is killing one person to save many lives, honestly it doesn't sound like the same level to me.