I always took the Batman and Joker rivalry to basically reflect the idea that the two of them are reflections of each other. In Joker's eyes, he cannot kill the Batman because the Batman is his main source of amusement. An antithesis of everything he isn't, but also someone that he can come close to empathizing with.
I'm kinda disgusted with the whole "gun" thing since I always took his attitude towards firearms to reflect his attitude towards never being like the people he fights against. Also, he never wanted to use the same sort of weapon that killed his parents off. I enjoyed that aspect of him, it is rare to have a hero subconsciously or willingly not do something or use something purely out of principle. Seeing that picture, I feel disgust and I really don't care to know more about the comic in general.
All fictional worlds have laws in them... set rules that the maker constructs and obeys. There are just certain things that the maker has to help distinguish their comic, book, or show from another. To go against a law, especially a hard-set one that spans into the basis of a character, that feels offensive.