I always found this to be one of the most interesting additions, and bits of dialogue, in the Snyder Cut of Justice League. I also rarely see it mentioned. Batman making a promise, on behalf of Harley Quinn, that he will kill the Joker. The tension of the moment and Joker’s reaction to this mention of Harley as well as what she wanted to happen to him were both things that made me interested in knowing what brought this about.

But, I suppose we will be left to wonder forever.