By the same argument, its the direct fault of the Police, Hospitals, Arkam, Harley Quinn, any hired goon, and every single citizen in the city - especially the victims and hostages, to rise up as one and universally declare Joker an enemy of mankind and attempt his violent death whenever he shows his pale face, even to the death of those attempting.
If people in position to kill Joker, legal or not, fail to then they are at fault for his crimes.
Batman might be the most equipped to catch the Joker, but that argument still holds for every other person in Gotham. Batman's ready and willing to serve the Joker up to the courts every time. The courts fail.
Predictable, but not Batman's fault.
Joker is kept in a barred wall cell during holding, under armed guard. Any guard could draw, fire and kill the Joker. The clever could even make it look like a failed escape plan, self defense, be a 'hero'.
This means someone else is held responsible for not killing Joker, one who has no 'code'.
This is a variation of the 'How does Superman chose who to save?' question.
But we agree on one subject. 'The Joker Will Escape Again'.
You say 'Joker killed, you threw him in jail, how dare you not kill him and prevent the next attack?'
I say 'Joker killed and was caught, thank god he can't keep killing.'
You use the Doylist argument that 'The Law in DC and the Citizens will always fail to serve their own interests properly, leaving Batman to be the only actor to the collective good. He MUST kill.'
If Batman in the narrative believed that, had that lack of hope that Gotham would never get better unless he committed mass murder of the rogues and mafia, then he'd have given up and joined the League of Shadows ages ago.
Keeping the criminals in line is Batman's job. Trying to fix the system is Bruce Wayne's.
I do get your frustration, and if I was a Citizen standing behind police tape - a .38 in my pocket and the Joker, chained to a stretcher in view? I'd shoot. And I'd use most of the rhetoric we've used to justify it.
But that's a personal thing. Besides, no one says Batgirl, any Robin, or any of the other Bat family should pull the trigger on Joker. They are their own people, not tentacles extruding from Batman's Aura. If we're blaming Batman for being the Superhero in place to catch Joker, what about EVERY OTHER HERO IN GOTHAM?! Hell, Huntress would be enthused at the opportunity.
If other superheroes are also guilty of the supposed crime, are they all guilty and suffering from a weirdly focused version of the Bystander effect or is Batman uniquely guilty because his name's on the book?
Yes, that is exactly what I am saying. It IS the direct fault of the Police, Hospitals, Arkam, Harley Quinn, any hired goon, and everyone else too. They have the power and the responsibilty to stop him from kill8ng people, yet they choose to do nothing. Again, with great power comes great responsibility, and those who have the power to protect the citizens from the monsters who threaten them have a responsibility to take care of the monsters, The only way to stop The Joker from causing people's deaths, directly or indirectly, is to finish him off once and for all. Yes, people will die if they try to kill The Joker, but they will also die if The Joker lives too, so either way, people are going to die, And if people are going to die no matter what, they might as well make sure that the clown is among the dead bodies rather than the one making the dead bodies.
Also, I call bull on your League of Shadows argument. Even if Batman started killing his villains, he wouldn't join the League of Shadows. For one thing, even for lethal anti-heroes like The Punisher or Red Hood, The League of Shadows is far too extreme by their standards, and Ra's Al Ghul is far too psychotic to be trusted. I'd even go as far as to say that Ra's is just as crazy as the Joker, just a different flavor of craziness. If anything, a murderous Batman would definitely kill Ra's and disintegrate his corpse to ensure he actually stays dead.
And no, I also don't believe that Batman is uniquely guilty of this, I was just using him as an example because he's the most egregious example of the lot. The argument I presented can apply to most superheroes who have this issue too.
Also, when you say "the Joker killed and was caught, thank god he can't keep killing", you're completely missing the point. No, even if you catch the Joker and lock him up, that will not solve the problem. It will NEVER solve the problem. Arkham Asylum cannot hold the Joker to save its life. In fact, we may as well rename it to
Arkham Hotel, because at this point, it's more of a place for Batman's rogues to rest and relax between their plans. They can sleep in and are well fed and taken care of, and they can leave whenever they want. Again, like a freaking hotel.
Plus, you say that fixing the system is Bruce Wayne's job, but let me ask you this: In all of Bruce's years that he's spent trying to use his money to fix the system, has that ever actually worked? The answer is a big, fat NOPE. Gotham has proven time and time again that you can't fix the system by throwing a bunch of money and influence at it. It was a corrupt dunghole on the day that Bruce lost his parents, and it's still a corrupt dunghole today.
And lastly, yes, I do believe that almost every other hero should be held responsible for the actions of their recurring villains too, not just Batman. In the Bat-family, Red Hood is the only one who actually makes sense, and the rest of them are crazy. I was just using Batman because he's the most egregious example, but he's not the only one this applies to. I believe that other supervillains like Lex Luthor, Sinestro, Reverse Flash, Black Manta, and the rest of the Legion of Doom should have been slaughtered like cattle a long time ago, and that their respective Nemesises should have executed them. I said it once and I'll say it again: Those who just stand by and let villains do whatever they want are no better than willing accomplices.
That being said, I understand that every rule has its exceptions, and I should also point out that I don't hold all superheroes responsible for this, nor am I always against the heroes adopting a no-killing rule, because there's a few superheroes who, in my opinion, can actually make the no-killing rule work, and usually have very good reasons for not killing their villains. One example I can think of is
Miraculous Ladybug, in which the no-killing rule actually makes sense, because almost all the supervillains in that show are innocent civilians who have been empowered and brainwashed against their will, so killing them would be horrifying. Not to mention that a lot of those brainwashed supervillains are close friends and loved ones of the heroes, so expecting the heroes to use lethal force is just unfair and unreasonable. Another example I can think of is
Danny Phantom, in which the no-killing rule debate is completely moot, because about 90% of Danny's enemies are ghosts, so they're already dead to begin with.