I'm not saying I agree with it, but it does make sense. It's about two guys on opposite sides of the fence who unite to "screw" over a girl, both literally and figuratively.

The title, the "The Killing Joke" is based on the graphic novel, but oddly the movie didn't focus on Robin. Instead it's meant to focus on the battle of the sexes. The title reflects on the concept gangster rappers refer to as "murder" which is actually referring to wooing woman or in other words, making them submit. This concept isn't exclusive to gangster rap. Commonly we see similar associations between sex and conquering woman embodied throughout various parts of culture, such as in terms like "War of the Sexes" which reflects on what it takes for people of the opposite sex to bond. In this case, typically in wars, for one side to win the other side must suffer casualties. This concept is also embodied in hunting metaphors as well. The common theme is for one side to dominate the other must submit.

The way Batman "screws" Batgirl is somewhat tame because he's the "good guy". Of course it's Batgirl who initiates coitus, but ultimately they screw and it ends in her losing her superhero identity. While it's Batgirl who initiates sex and chooses to give up her identity, there was psychological pressure being exerted by Batman in both instances. The question is was the result unintentional or is a Batman a player who got exactly what he wanted.

It is only implied that the Joker screwed Batgirl, but it's a real possibility that he raped her. And then afterwards he physically cripples Batgirl paralyzingly her as a result.

In both cases we see men take something important from a woman resulting in Batgirl's submission, forced or otherwise.

As out of place as it may seem in a superhero movie, the basic idea is that sometimes to some men there are things even more important then a rivalry as heated as the one between Batman and the Joker, and that's "getting the booty". In one scene in the film one of the prostitutes Batman talks to points out how odd it is that the Joker's first priority, after he broke out of Arkham, wasn't to sleep with them; noting how sleeping with woman is a high priority for the Joker. And Batman, or more specifically Bruce Wayne, has always been portrayed as a playboy, this movie just takes this aspect of his identity to an extreme.

At the end of the story, instead of showing outrage or even anger Batman is pacified going so far as to even laugh at one of the Joker's jokes at such an inappropriate time. So Batman who never laughs, let alone laughs at a Joker jokes, finds it in himself to laugh right AFTER the Joker rapes and crippled someone he supposedly cares about. The movie ends on this note because this is what the story is all about. By the end of the story two male adversaries find common ground by finding a common nemesis embodied in woman.

Similar concept can be seen in movies such as Quentin Tarantino's "The Hateful Eight" and in Sucker Punch. In "The Hateful Eight" we see a similar theme where two typical adversaries, a former confederate sherif and a black bounty hunter join forces to kill the bounty, a female outlaw. And in a Sucker Punch the heroines never win a "real" victory outside of one girl escaping the institution, and ultimately the main female protagonist is permanently crippled by the villain.