Don't get me started on the Joker. The problem as regards him not being dead come in two varieties.
Extra-legal retribution: The list of people Joker has killed is immense. The list of people in the DCU who are powerful and/or wealthy yet not burdened by moral scruples is also very large. Yet these two lists have never intersected? Joker's never done up the relative, friend, S.O. of any other supervillain who'd be pissed off enough to make a run at him, either singly or with friends? Joker's never whacked a relative of the mob, or any rich guy who happens to know the phone # of any one of the DCU's various elite mercenary assassin types?
Seriously. Joker makes it his mission in life to drive as many other people as possible to the depths of rage or despair. You'd think at least one of them would have the $10 million or so necessary to pay Deathstroke's tab. Or David Cain's, or Lady Shiva's. (The latter two in particular -- even if you grant that the Joker is a formidable enough opponent you need to be Batman to defeat him, the last two names on my list have both one-upped Batman at least once.) Seriously.
And really, I can understand why Lex Luthor lets the Joker live: a Batman without the Joker in his life is a Batman who has enormously more free time to roll on down to Metropolis and give his buddy Kal-El a little help in unravelling Lex's labyrinthine business schemes, which is a headache Lex Luthor knows he does not need. On the few occasions Lex and Bruce have gone at it as dueling Magnificent Bastards, Lex's life royally sucked. Lex does not desire repeats. So the Joker's existence is significantly useful to Lex Luthor. But Lex? You're about the only guy in the DCU supervillain community who'd agree with you on that point!
So why hasn't the Joker accidentally fallen headfirst into his mirror while shaving one day, to end up suffocating on the ass end of Mars? Or accidentally on purpose being transmuted into a pile of arsenic? Or sniped in the face from a mile and a half away? Or buried underneath his own personal miniature glacier? Or found mysteriously strangled by animated vines and a 100-foot tree jammed up his anus?
Legal consequences: Let's leave out the vast majority of Joker's crimes, even though just 1% of that list would be considered a great year for Jason Voorhees. I'm gonna just name three things right off the top.
* Attempted to kill the UN General Assembly, in session, with nerve gas. Stopped literally in the act (with the gas safely inhaled by Superman and then super-breathed out into space) by the World's Finest. ("Death In The Family")
* Attempted to destroy New York City with sea-launched cruise missiles mounting nuclear warheads. Stopped by the Birds of Prey. (BIRDS OF PREY #16-17)
* Released a biological warfare agent that mutated dozens of supervillains into crazed killing machines. Summer crossover event spanning many titles. ("Last Laugh")
What is the common factor among all the three above events? They involved terrorist attacks with weapons of mass destruction.
Yup, that's right. The Joker is the only living person in the DCU to have attacked the American mainland with every single type of WMD. Nukes, chemicals, germs, he's used them all.
And they still treat him as a criminal case? Why is he not the DCU's equivalent of Osama bin Laden at this point? Why does his showing up anywhere not result in people saying things like 'terror alert to red', 'mobilize the National Guard', 'lock down the city', 'NEST teams on standby', 'CDC biowar response teams on standby', 'Delta Force has been given orders to terminate', etc? Where the hell is the Suicide Squad when you actually NEED them?
(Edit: And I will register my annoyance here at that goddamned "Salvation Run" mini where the Suicide Squad was in fact secretly abducting supervillains... including the Joker... and then teleporting them to an alien prison planet where they had every opportunity to escape from, especially since they were dumb enough to send Lex Luthor there. Yes, send the most brilliant criminal genius in the DCU to an entire planet full of angry supervillains for him to lead. That couldn't possibly go wrong. No need to send Lex to a different planet, all by himself, so that he has nobody else's superpowers to leverage off of to make up for the lack of technological resources he'd need to build a way back. Its just... um, if you're willing to go so monstrously illegal as to kidnap people and exile them to another solar system for life, without trial, why not just teleport them into the sun? It didn't even make any sense by evil logic.)
So yeah, allow me to register my own annoyance at a comic book trope: in the comics, far too often indiscriminate mass murder and attempted mass murder of zillions is treated the same way as would be a simple homicide during an armed robbery: with measured law enforcement response and scrupulous attention paid to your civil rights. As opposed to people actually acting like real people would react to a dude who's tried to use nerve gas to kill the entire United Nations building, or blow up NYC with a neutron bomb, or unleash super-plagues. Hell, as seen in the 'Salvation Run' example above, even when they do throw the rule book out the window, they do so in the stupidest manner possible, melodramatically tying villains to train tracks and twirling their mustaches instead of just shooting them.