I love how people keep talking about taking a gun to him. I'm going to laugh if someone like the Joker does appear, the people who said they would shoot him, find themselves in a spot to shoot him, and don't. Saying you can kill someone, and then doing it, its a compeltely different matter. When it comes down to the point of actually shooting, or killing, someone, you get a lot of emotions at once. Most people are overwhelmed by it, especially the ones who think they can shoot people easily.
Now, on topic, if someone like the Joker was real, we'd all be screwed. His intellect would run us all into the ground. That is the Joker's superpower, his mind. He can fuck with people's heads easily. He can make people do what he wants, without them knowing. Now, of course if the Joker was real, he would also know about the movies, know that Convenience was there and such. He wouldn't rely on luck. Now, people may say, oh but he got lucky, how did Batman know he would go after the girl? How would he know that Harvey wouldn't just blow his brains without flipping the coin? ITS CALLED PSYCHOLOGY DUMBASSES! If you know the way humanity works well enough, you will know how each person will react. Joker studied these damn people. It was obvious how he knew each move they would make, he knew that Batman would go to the damn girl because he knew how he reacted to her. If you jump out of a building just to save a girl, you are going to go run to her over some guy she is dating. It was the sign that Batman was corrupted by love. He choosed love over peace in Gotham City, THAT was what Joker was trying to prove to Batman. He also almost got through to Batman, but of course, good guys in movies always win.