The Time Joker Refused To Work With Red Skull
The Joker might be a merciless psychopath who has committed unforgivable crimes like beating a teenager to death with a crowbar, crippling a young woman just to drive her dad insane, and plagiarizing David Bowie's fashion style, but even he agrees with the supposed German saying: "If there's a Nazi at the table and 10 other people sitting there talking to him, you got a table with 11 Nazis." Or, at least, a couple of versions of the Joker do.
In the '90s, when DC and Marvel were still on speaking terms and let their characters play together, they released a Batman/Captain America crossover in which the heroes team up to foil a plot to steal the world's first atom bomb. Oh, yeah: this comic is set in 1945, in a parallel reality in which Superman and Batman debuted in the '30s and aged in real-time (an idea DC almost expanded to their entire line recently, only to get cold feet). This evil plan is carried out by the Joker and the Red Skull until Joker finds out that his partner is a legit Nazi and not just cosplaying as one.
Regular mass murder is fine with the Joker, but racist mass murder? F#@% off. Once this depraved killer makes it clear that he's not working with a bigot (especially a red one), the two villains immediately try to kill each other with their signature poison attacks ... only to find out that the toxins are too similar and cancel each other out.
Later, the Red Skull manages to get a hold of the atom bomb, and Batman is unable to stop him from dropping it near Washington D.C. -- but, luckily, the Joker shows up and delays the Skull long enough for Captain America to pilot the plane to the Atlantic ocean, where both villains fall with the bomb. Yes, the Joker saved America out of pure, disinterested patriotism, and how did Batman thank him? By rejoicing that "we've seen the last of" him.
Now, like we said, all of this happens in an alternate reality, but there's at least one other version of the Joker out there in the multiverse who shares this one's aversion to swastikas. In 2020's Injustice: Year Zero #4, the same Joker who tricked Superman into killing all of Metropolis in this reality reaffirms that he loves America as much as he hates Hitler's ideas:
On the one hand, pretending that a character who has tried to destroy all of reality at least once would give a crap about Nazism one way or the other is kind of a silly idea, like that time Marvel made Doctor Doom shed a tear over the victims of 9/11. But, on the other, we'd absolutely read a series that's just the Joker slaughtering white supremacists in inventive ways every issue. Hey, if there's one where Joker raises baby Batman, anything's possible.
Top image: DC Comics/Marvel Comics