There’s a question about the rivalry between Batman and the Joker which mostly concerns diehard Batman fans. However it is also pertinent and i’d say the very first lesson in understanding ethics. Before heading to the question, let me first give a brief primer about Gotham and the Batman universe, a little more than Nolan’s movies. As many of us know, Joker is Batman’s arch-nemesis who’s done many horrible things. In the movie he kills a number of people including Rachel, turns Dent into Two face, blows up a hospital etc. In the universe beyond The Dark Knight, he’s killed a number of people including school kids, tortured Robin, Alfred, Jim Gordon’s wife etc. This is purely the mark of the greatest fictional villains of all time. However, spreading chaos is just a means to an end for him. His simple objective to blow shit up is to get the batman to kill him. The brilliant truck flipping scene in The Dark Knight is followed by him standing in the middle of the road and mocking Batman to kill him, at which point Batman veers off and crashes into a wall.
So the question is, “Why doesn’t Batman just kill the Joker?” It makes total sense, the person responsible for the death of so many innocent people and having the potential to inflict many more such wounds on Gotham doesn’t deserve to live, right? Wrong according to Batman. Because Batman only has one rule. He doesn’t kill. That’s it. Now, there are so many justifications which say Batman is scared about his own mind as believes him and the joker are the same. That once he kills Joker, he will not stop at it. He would go down the slippery slope and would end up becoming a similar version of the Joker himself. May be, but it has got to do a bit more than just Batman’s psychology. Or us looking at the slippery slope as a linear path of rationalising our judgements.
The important thing here is the rule. Being incorruptible for Batman is to not break the rule at any cost. Because killing Joker means destroying what the idea of Batman stands for. Joker knows this and hence all the attempts, taunts and the mocking. The point here is not whether killing is good or bad. It is about having a rule, and what it takes to stand by it. When we look at it that way, the slippery slope stops being a linear scale and ends up binary where darkness is a void which pulls you like gravity.
When we look at morality in the world, the good and bad, we often wish for more clarity in distinguishing the good from the bad. This is what takes us towards religion or patriotism. But sadly they are often not enough. Easier way to understand morality would by looking at good and bad for what they are, subjectively. Having your own rules, any rules. And once you have them, what does it take for you to stand by them when you encounter your Joker? Will you be immovable when an unstoppable force meets you or will that little push be just enough?