The Joker is completely straight-faced in The Killing Joke during the moment he's trying to actually reason with Batman. Later on, when Batman in turn tries to reach out to him and offers him rehabilitation, the Joker can only look at the man dressed up as a giant bat in stunned, incredulous silence before solemnly apologizing and telling Bats that it's "far too late" for him to redeem himself now.
The Killing Joke ends with an OOC moment for Batman. Long story short, Joker does some awful, awful things, Batman catches him, Joker tells him a joke. Nothing unusual so far... But then Batman starts laughing... and laughing... and laughing. No wonder plenty of people theorize that Batman finally straight-up kills the Joker in this moment.
In The Nail, not even Batman was able to maintain his cool before the Joker's extremely despicable deeds and ended up outright killing his archenemy (something he would otherwise absolutely avoid).
Death of the Family: Joker has always used Joker Venom, which kills a person and leaves them with the creepiest grin on their face. But this time, his venom kills a person, and leaves them with a frown on their face. Something is wrong...
The Joker has always had a fondness for jokes, tricks, and gags in his various schemes and murders, but in A Death in the Family he forgoes his usual methods and simply opts to beat Robin (Jason Todd) half to death with a crowbar and let a bomb finish the job.
Joker: It should have been funny... but it wasn't.
While Dick Grayson was the sole Batman in the DCU, he let security cameras record him beating up thugs, so that criminals would know that Batman was back in town. However, Two-Face sees him smiling in one of these recordings, and this tips him off that the Batman he's watching is not the Batman, as Batman does not smile.
Similarly, during Knightfall, Joker takes one look at Azrael-as-Batman and realizes it's not the real Batman, walking away from the fight. Commissioner Gordon, Catwoman and Superman all take similar notices, Gordon in particular getting his first hint when he turns around mid-conversation and sees that Batman hasn't abruptly disappeared.
In a tie-in issue to Justice League: Generation Lost Dick Grayson (who had been acting as Batman) is under the same mental compulsion as everyone else to forget Maxwell Lord exists. That includes believing Ted Kord committed suicide. Bruce gets Kord's body, pointing out how there was no gunpowder residue and how the bullet hit dead center in the forehead. Every time he points something out, Dick comes up with an explanation...until Bruce points out how Dick is going out of his way to look at any explanation, no matter how unlikely, rather than Ted being murdered. The realization he's going against everything Bruce trained him to think is enough for Dick to break through the mental conditioning and accept Ted was murdered.
Invoked and weaponized in Dark Nights: Metal — The Batman Who Laughs can prepare for every possible strategy or event that Bruce's mind can come up with, so it takes something incredibly out of character to catch him off-guard, namely Batman teaming up with the Joker against him.
In the story "Broken City", during Batman's investigation into what he believed to be a series of murders, he pays a visit to the Ventriloquist and his puppet/split personality, Scarface, where in a strange turn of events, after Batman mentions the suspect's name, Angel Lupo, Scarface, who usually has a Hair-Trigger Temper, who was the calmer of the two and it was the normally-shy and calm Arnold Wesker, who flew off the handle into violent anger. At the end, it's revealed that Elizabeth Lupo, Angel's sister, had a fling with Wesker, wherein she got pregnant with his child and like many people, Batman included, Wesker wrongly believed Angel killed her. Furthermore, when the Scarface personality mocks Wesker for "not keeping it in his pants," Wesker tells him to shut up and attacks the puppet, destroying it.
Final Crisis saw Batman, in the one time to date (not counting his very early comics and alternate universes) that he's actually used a regular gun on someone to kill them, was with Darkseid, because the situation was that serious. Nor was he the only one, as Wally West and Barry Allen led the Black Racer to Darkseid and even Superman tried to kill Darkseid, being the one to actually deliver the death blow. That's right: things were so bad, four members of the JLA, who usually uphold "Thou Shalt Not Kill", actively made attempts to kill the villain.
There is one scene in which Delirium pulls herself together. Delirium, as her name suggests, is the Anthropomorphic Personification of insanity. She even comments that it hurts to be sane. ("I know things.") And from a brief moment of chilling clarity in Brief Lives to her brother Destiny:
Delirium: Do you know why I stopped being Delight, my brother? I do. There are things not in your book. There are paths outside this garden. You would do well to remember that.
Later in Brief Lives, when Dream realizes that Destruction has left them behind forever and that the price of the search now has to Mercy Kill Orpheus, he has a few moments of very uncharacteristic kindness for his servants, several of whom begin wondering if he's all right.
When Supes gets pissed, which is rare, beware, because he is about to test his Thou Shalt Not Kill credo.
In Superman's Return to Krypton, Martha is shocked when nice, quiet Jonathan Kent decks a crook who tried to kidnap her and hold her hostage.
In Man of Steel (2018), Superman suddenly gets a emergency signal from the Fortress of Solitude and quickly leaves, not even telling Batman and a firefighter who are investigating an string of arsons in Metropolis goodbye. As Batman points out, if the most friendliest man in the galaxy leaves without saying goodbye, something is very wrong.
Any time that the Pre-CrisisLex Luthor, the version who used to be Superboy's friend and a pretty nice guy, would have a Pet the Dog moment, it represented his earlier decent nature showing through, and (rightly or wrongly), it gave Supes hope that someday he might come back to the side of the angels. One of the very last Pre-Crisis stories, Superman (Volume 1) #416: "The Ghost of Superman Future", indicates that in at least one possible future, Lex did eventually reform and they became friends again in their old age. It was by Elliot S! Maggin, who was very fond of Luthor. He later wrote a short story along similar lines called "Luthor's Gift."
Also, any time Luthor feels the need to actively help Superman is a sure sign that things are at their lowest ebb for our hero.
In Superman vs. Shazam!, Supergirl can tell something is wrong with her cousin when she finds Superman raving about killing Captain Marvel. Similarly, when Mary Marvel listens to her sweet, would-never-hurt-a-fly brother Billy shouting that he is going to Earth-One to kill Superman, she gets worried and takes off after him.
In The Leper from Krypton, the Justice League hold a private funeral for Superman, and the Flash notes even their stoic and quiet teammate Batman is crying openly.
The 1991 War of the Gods event has the various pantheons fighting it out with the Greek and Roman gods ready to ravage Earth. At the height of the battle, Ares appears alongside Mars and their various counterparts, to relate that Circe is manipulating this entire conflict. The fact that the Gods of War are the ones calling for peace is enough to convince both sides of the truth.
Rorschach is gruff, insulting, and arrogant most of the time, and also doesn't usually use pronouns or proper names. When he says, "Daniel, I apologize. I know it's difficult being my friend.", you know he's being dead serious.
When Dan/Nite Owl finds out that his friend and mentor, Hollis Mason/the original Nite Owl was beaten to death by members of a gang, he starts beating the ever-living crap out of a gang member, to get information about the murder. Even though Nite Owl has no aversion to violence, the raw viciousness makes Rorschach of all people stop him. (Although his rationale is to tell Dan, "Not in front of the public.")
Wonder Woman has her own version. When she puts aside her lasso and other tools meant to disable and actually takes up a sword or ax, or both that time Medusa challenged her after killing a child in front of her, it means she has declared war and someone's going down — or possibly some army. It doesn't really apply to the New 52 version who's, well, sword-armed almost all the time, but it used to be a big deal for the champion of peace to take up a weapon meant to kill.
