There isn’t any fictional relationship that makes more sense to me than that of Batman and The Joker. One of the main reasons I love Batman is because of his relationship with The Joker. One of the main reasons why I love The Joker is because of his relationship with Batman.
Life had kicked him to the curb and walked away from him at his worst, letting him rot on his own with his dead parents laying lifeless next to him. On that day he had two choices; he could say fuck the world and burn it to the ground just like it did to his life, or he could make it his life goal to restore justice in the city the has given him nothing, but has morphed him into who he is.
Then there is The Joker, who just like Batman, was one day dealt a shit hand. Just as Batman had lost his entire world in a split second, the same thing happened to The Joker. In a split second he not only lost his wife, the love of his life, but also his unborn child inside of her, was killed by a cooking accident or set on fire by a corrupt cop. Either way you paint it, everything he had ever cared about in his life was gone in an instant.
In one second, just as Bruce Wayne did, he lost the only two things he has ever loved in this world. Gotham kicked both of them to the curb and left them there to die. The same as Bruce Wayne, who took his worst day and transformed it into his best day, The Joker turned his tragedy and had it define who he was. Unlike Batman he didn’t care to restore order. Order was exactly what caused this to happen to him. Instead he devoted his life to prove how corrupt our world was… Just as Batman was.
There is no difference between Batman and The Joker’s. There’s no distinction between them from any of us. The same as they both faced their worst days, fell to rock bottom, they were left with two options; to keep going and make the world a better place or exploit it for all it is, flaws, imperfections and all.

The same internal conflict we all deal with, one I’ve been continuously finding myself battling. It’s just as easy as it is as difficult to make the decision. Are you more like Batman or more like Joker? How we react to these moments in our own lives, when we hit rock bottom, the decisions we make are what define us indefinitely.
Batman will never kill The Joker, they’re far too much alike. One choose chaos and the other ran to justice. It’s the same internal conflict we all have. We all have this demon or little voice inside us telling us that doing wrong is right, but we also have this little light inside of us, reminding us that the light will forever shine in the darkest of times. 
I love Batman and The Joker for far more than just The Dark Knight. I love them for their relationship in The Killing Joke, or countless times in their comics where they’ve understood each other. How they’re not so different after all. I love the movie The Joker because it depicted exactly what goes through our minds on our worst days. Bruce Wayne is just the epitome of what happens when you follow the light. And if you ask me, I don’t think either of them are wrong for the path they’ve chosen.
That’s why Batman will never kill The Joker. He gets where he is coming from. They were both dealt the same hand of cards and deep down Batman, since he’s been there at rock bottom, he thinks he can show Joker the light. But we all know far too well, how hard it is sometimes to see the light. I mean how would you react if you lost everything you’ve loved in the blink of an eye?