This is based on opinion. Please don't list it on a work's trope example list.
Being an R-rated origin story about the most dangerous enemy the Dark Knight has had to face, it should come as no surprise that
Joker has disturbing moments, on a physical and emotional level.
- When Arthur's psychiatrist asks him if he was better off at the psych ward, we Smash Cut to Arthur banging his head against the door to his room in the ward.
- When Arthur's boss tells him unless he brings back a sign he lost or he'll take it out of his paycheck, the boss's talking goes inaudible as Arthur sports a silent, creepy smile.
- The subsequent image of Arthur angrily stomping and kicking on garbage outside.
- Arthur stalking Sophie after she dropped her daughter off at school. Perfect delivery of Paranoia Fuel.
- The first people Arthur hunts down are the three WayneTech employees. While he killed the first two out of clear self-defense, he goes out of his way to pursue Ryan — even as he's running for his life — and shoots him dead in the subway, in total silence. And then, even that isn't enough: he then shoots the corpse multiple times out of cold malice.
- Afterwards, the weight of the killings hits Arthur and he runs through the city in a panic, eventually finding refuge in some dirty bathroom. Instead of curling up and crying or even trying to rationalize his actions, Arthur just starts dancing in slow motion while some eerie music plays.
- The scene where Arthur stops by the Waynes' house and interacts with a young Bruce Wayne. While Arthur is nothing but nice to Bruce at the start (believing him to be his half-brother), he's uncomfortably touchy-feely with the poor, confused kid, shoving his fingers in the boy's mouth to literally force a smile on him. Given that we've already seen Arthur's mood swings, we the audience know Arthur's already strange affection could turn to violence if provoked.
- When Arthur visits Arkham Asylum hoping to find any records regarding his mother. The clerk comes up with a file detailing how Penny Fleck was found guilty of child endangerment. When the clerk is reading this, he has this sudden realization that the child in question is in fact all grown up and standing right in front of him. Not helping is the fact that Arthur was acting even more erratic than usual, certainly an indication that his sanity has worn from the start of the movie.
Arthur: Last time I ended up taking it out on some people. I thought it was gonna bother me, but it really hasn't.
Clerk: What's that?
Arthur: I fucked up! I did some bad shit! (chuckles)
- At his wit's end, Arthur decides to kill his own mother by means of Vorpal Pillow.
- What makes it especially horrifying is how he doesn't react in the slightest while he smothers her to death, and how there's no backing music at all in the scene; we only hear the sound of them struggling, his mother's muffled cries, and the increasing tempo of her heart monitor until it just... stops. True to reality, there is no dramatic flatline, just absolute quiet.
- Special mention to just how ghastly her face looks after he finally lifts the pillow...
- There's also the line he says before he does the deed...
- With the reveal that he imagined their entire relationship, Arthur coming into her apartment unannounced is this to Sophie and the audience. She's a single mother who has just put her young child to bed, only to find a man she's only spoken to once silently sitting in her living room, having broken in without her noticing. All she can do is calmly try to ask him to leave only for him to gesture shooting himself in the head, clearly terrifying her.
- Arthur killing Randall is easily the most brutal and violent scene in the whole film, and that’s saying something for very good reasons. He jams a pair of scissors into his jugular vein, then his eye, then proceeds to slam his head against the wall until the blunt force trauma kills him, all while Gary is wailing for him to stop. At least the Wall Street three and maybe Penny could be seen as Asshole Victims; while Randall did give Arthur his gun and subsequently threw him under the bus when he got fired for it, that comes nowhere near deserving of a death this brutal.
- It certainly doesn't help the fact that at that moment, Arthur's white-painted face (looking near-identical to the Joker in the comics) is streaked with blood (pictured).
- Immediately following this is Gary trying to escape the apartment, being forced to cross Arthur and Randall's corpse, but being unable to reach the lock, with his only course of action being to ask Arthur for help. The scene is hilarious in an especially horrible way, but it's still terrifyingly intense, as Arthur has just become so unpredictable that him also murdering the completely innocent Gary is presented as a real possibility (though fortunately, he's spared).
- Once he gets to the Murray Franklin show, Arthur stays backstage and silently smokes a cigarette while Murray once more plays Arthur's disastrous stand-up routine to the audience. His gaze is burning, and the audience is left with a deep unease as the wheels silently turn in Arthur's head.
- Murray Franklin's death. After Joker rather gleefully admits to the three Wall Street murders, Murray rips him a new one, stating that not everyone is a horrible person and that his actions were utterly unjustified even if that were the case. Joker just laughs and says "I know", then telling a final joke despite Murray's protests — slowly raising his voice until he is resorted to tearfully screaming at him, and the moment he finishes... Boom, Headshot! On live television.
Joker: What do you get... when you cross a MENTALLY ILL LONER, with a SOCIETY, THAT ABANDONS HIM, AND TREATS HIM LIKE TRASH?! I'LL TELL YOU WHAT YOU GET! YOU GET WHAT YOU FUCKING DESERVE!!!
- The chuckle Joker gives to himself after shooting Murray while his audience screams in the background, especially considering the implication that it's a rare genuine laugh, and not one that's faked or triggered by his condition.
- The very chilling shot of Arthur finally seeing that the citizens of Gotham do not care for him, but for the persona he accidentally created. His response? Painting a smile on his face with his blood, all while relishing through tears his rebirth as Joker.
- The final scene at Arkham is an Ambiguous Ending at best, though one of the most disturbing factors would have to be Arthur's demeanor throughout it. Arthur is laughing his ass off at a "joke" he never tells - and this is a genuine Evil Laugh as opposed to the neurological 'laugh' he lets out throughout the film. His voice drips with joyous malice that Arthur never expresses in any other scene. One of the few solid facts given by this scene is that there is nothing left of Arthur Fleck. As far as he was concerned, Arthur never existed as he wasn't recognized. In his place, there now only exists the making of a violent, sadistic, and what has been interpreted as thoroughly evil Monster Clown with not even a shred of ethical goodwill left, who definitely finds mass homicide to be mere comedy in his eyes. There now only exists The Joker. And this is before he happily walks out of the room, leaving bloody footprints on the tile and dancing to Frank Sinatra’s That’s Life.
argues that even if Arthur killed Sophie, the fact that it isn't shown keeps him sympathetic, but it can also be a effective example of how leaving some things to the imagination is much more effective than confirming them outright.