WARNING: The following contains spoilers for Batman/Superman #3 by Joshua Williamson, David Marquez, Alejandro Sanchez and John J. Hill, on sale now.
There have been many evil versions of Superman over the years who've turned their back on humanity or embraced the cruelty of Superman's villains as their own. But while totalitarian Superman or zombie versions of the character are frightening in their own way, they have nothing on the concept of a Superman who's been Jokerized.
Introduced in Batman/Superman, a Superman who's become infected with Joker Toxin would be one of the single most dangerous things in the multiverse.
ONE BAD DAY
The Batman Who Laughs is revealed to have infected various heroes across the DC Universe with a strain of the Joker Toxin. This will bring out the worst aspects of each of the heroes, turning them into dark reflections of themselves. His plan included infecting Superman by using an already-corrupted Shazam to surprise him. But working with Batman, Superman is able to escape the battle unscathed and uninfected.
However, the pair see an opportunity to try and steal information out of the Batman Who Laughs by faking Superman's full-blown infection. So, Clark takes just a bit of Joker Venom and Superman is briefly corrupted into a darker version of himself.
Superman seemingly frees the Batman Who Laughs, and the pair cackle as they prepare to start killing their way through the DC Universe. But Batman Who Laughs is able to quickly tell that this is a hologram and that Superman isn't quite as corrupted as he'd hoped. So he goads Superman into lashing out, almost giving in to the Joker Toxin that is pumping through his veins.
Batman is able to contain Clark, though, and hold him down long enough to get the cure into him. He even makes sure to put Superman under intense sun lamps to help burn out the rest of the infection while going to confront someone else the Batman Who Laughs infected.
This incident should be seen as a warning to everyone in the DC Universe: a Superman who's truly infected by Joker Toxin would be arguably one of the most dangerous beings in the universe. Even just lashing out momentarily, he almost murders the Batman Who Laughs and casually tore up sections of the Hall of Justice.
If Batman hadn't been able to stop him quickly, there's very little that a mad Man of Steel couldn't do. It would be a Superman fully unleashed on the world, which would likely end in countless scores of dead. He would probably even be happy to work with the Batman Who Laughs, forming the deadliest version of the World's Finest in the entire multiverse.
There's still some concern of Superman becoming fully infected, too. If the Batman Who Laughs was able to infect so many heroes, there's no reason to believe he couldn't recreate the effect down the line in some way. Even with normal Joker Toxin, he could break Superman. It gets worse when one considers that Superman didn't spend as much time as he should have underneath the sunlamps constructed by Batman. Superman believes he expelled the last of the venom from his body, but he may be wrong about that. The Joker Toxin might still be pumping through his veins, just waiting to transform him into the darkest possible version of himself.
With the battle against the Batman Who Laughs only getting worse as the other heroes succumb to his dark influence, there's always the possibility that Superman could end up turning on the heroes as well.