Joker wants Batman to kill him. If Batman kills Joker, he would be going against everything he stands for. it will also ruin Batman's reputation as a hero. That's what Joker wants; to ruin Batman's reputation and make him go insane. Green Goblin tried to force Spider-Man into doing the same thing.
" @Crom-Cruach: That was the point of his design. Deadpool's look was molded from the simplistic look of Spiderman. Being generic was what they were going for. As for Grifter, his mask only hints at DP because of the color. The open top, ripped bottom, and more circular black around the eyes is too off. Change the color scheme, and you wouldn't see the resemblance anymore. "
Even if they change the color scheme, there would still be a resemblance because they have similar tactics and personality.