I disagree with you because we do in fact execute mass murderers without due process if they merit it. Exhibit A: Bin Laden. That was a straight up assassination. Then there are all the drone strikes we have. Etc. Your also comparing Joker to a run of the mill murderer. We are talking about a guy with a body count in the thousands who has repeatedly escaped Justice and in the context of that world Batman is the only one capable of dealing with him in a way that could actually solve the problem. The institution is too flawed and it keeps putting him in a mental institution with lacks security, and no regular person is capable of catching Joker. So it goes from Batman, to the crappy Gotham system. I get the sentiment of a criminal who commits one l
It's in no way the same thing as saying a criminal gets off on a technicality. It would more be like a terrorist masterminds an attack that kills hundreds. Our Seals catch him. He goes through a court system, he is placed in a facility where his followers have no issue breaking him out. He escapes and commits another killing spree. That cycle repeats about a hundred times. At some point the people who get him before he goes through the system have the opportunity to decide to just bypass every regular form of due process to save lives. That's a little different than killing one guy who got off on a technicality. And the Joker is more akin to that example.
And it's one thing to say, oh you are the one taking it too seriously when it's just a throwaway line in a comic book. When you have whole Batman stories that center around the Joker trying to push him to that breaking point and he ends up not because of some crappy reasoning.
I have no problem with then saying it's just some mental hang-up Batman has, and that he just can't handle doing it. That's fine with me. Atleast it's more realistic than some of the reasonings they come up with to make it righteous that he's knowingly letting a mass murderer free to kill another day. Or even do what they did in the old days where Batman thwarts Joker and in the final confrontation he seemingly falls to his death and there is no closure. That is perfectly acceptable.
But in the context of everything that goes on on in those stories, when they force that issue in your face, and there are plenty of stories that make "why doesn't Batman kill the Joker" a big part of the story, and then you get a bunch of nonsensical ramblings that don't work when you put any thought into, it's just annoying and needlessly tears down the logic and distracts from the story.
And the Joker specifically has been used many times for this. We have a whole video game where the main premise is Joker commits a tragedy and Superman decides to kill him, and then the result is that world becomes an alternate universe where the heroes are tyrants. Thats a little bit more than just using it to build dramatic tension in a story.
It's just not well thought out logic that distracts from the story. And there is a reason why so many fans of Batman always ask why he just doesn't kill the Joker.