I finally got to watch Joker tonight and when he shot those guys on the subway, I immediately thought of Jeremy Christian on the Max. Jeremy has been saying for awhile that he only stabbed those dudes after they circled up on him and wouldn't stop putting hands on him even after he threatened them with a knife. That sounds a lot like what happened to The Joker on the subway. You can watch the scene via YouTube towards the bottom of this page. The scene clearly shows The Joker getting beaten up on the subway because a group of drunk Wall Street types didn't like him laughing non-stop. While he was getting beaten he pulled out a .38 and started shooting them.
I don't think anyone would argue that The Joker did not act in self defense. He was being beaten and did what he had to do to avoid further harm to his person. But what if The Joker did not wait for the beating to begin? What if he pulled the gun as soon as he thought they were going to smash him?
Jeremy Christian's case is just like that. The only differences being what came out of his mouth to motivate his opponents and the use of a knife instead of a gun. Had he waited to see if they were going to smash him on the Max and then started stabbing, everyone would call that self defense, but doesn't someone have the right to defend themselves before a perceived battery begins?
It seems like people are too pre-occupied with what they think was coming out of Jeremy Christian's mouth to care whether or not he was in fear of imminent bodily harm. All The Joker did was laugh non-stop and he got the crap kicked out of him, so perhaps he didn't think he was going to get beat until it happened. If he had been making a bunch of ethnically or religiously insensitive remarks he probably would have seen the beating coming like Jeremy did.
People get beat up for all sorts of shit talking all the time. So, if you're talking shit about just about anything to anybody and three random dudes circle up on you in public, you're going to fear for your safety. That is just common sense psychology.