So I'd probably be labelled as a casual to those super into the comics, but I think I've experienced enough Batman to know that his "no kill" rule does way more harm than good, and he should make regular exceptions.

The Joker is a prime example. Even in lighter works, he kills, seriously maims or otherwise damages things and people, sometimes beyond repair. He's taken Robins in multiple continuities, either killing or brainwashing them (not to mention crippling Batgirl). He corrupted Dr. Quinzel into Harley Quinn, and although she was susceptible to it in the first place if left to her own devices she would probably have just turned out like the Jew fro guy from TDKR.

Speaking of, he is responsible for 600+ deaths in that work, including gassing the audience and the mass shooting down at the carnival.

Even in works that only play at being serious, such as the WB animated series and its successor Beyond, Joker takes a different approach than the rest of Batman's rogues' gallery. While other super criminals like Mr. Freeze or the Riddler are less severe criminals, they also have very human motivations. The Joker does not, he's just insane.

So why doesn't Batman kill the Joker? The other villains would go on stealing diamonds and less people would die.