Having found legal documents from Arkham which confirm his suspicions, Arthur kills her while in custody of the medical hospital by smothering her with a pillow in revenge for all years of lies and the abuse during his childhood.
Joker's mom was mentally ill and was delusional. This was reflected in how she raised Arthur; it explains why HE was as screwed up as he was. Her delusions were becoming a problem. That's why Thomas Wayne had her committed, not because he had an affair to cover up by hiding her in a mental institution.
When Arthur gets assaulted by a group of teenagers, Randall offers him a gun to defend himself, but when Arthur gets fired for possessing the weapon, Randall gives no help to him and demonstrates his true colors and gets killed by the newly turn nihilistic criminal Joker.
Conclusion: Penny did in fact have an affair with Thomas Wayne however they made her sign an adoption form for her own child to cover it up. We know she signed papers and we now know she did have an affair with Thomas Wayne.
The psychopathology Arthur exhibits is unclear, preventing diagnosis of psychotic disorder or schizophrenia; the unusual combination of symptoms suggests a complex mix of features of certain personality traits, namely psychopathy and narcissism (he meets DSM-5 criteria for narcissistic personality disorder).
The heist goes awry; the comedian leaps into a chemical vat to escape Batman, surfacing disfigured. This, combined with the earlier accidental death of his wife and unborn child, drives the comedian insane, turning him into the Joker.
To make this happen, the most famous clown in comics persuades the wizard Zatanna, however, she wants nothing to do with the Joker and she casts a spell but it backfires and he is the one that becomes pregnant.
Arthur, as a result of the constant abuse and trauma he had endured, grew up to forget that Penny was apart of this abuse and this resulted in him growing up with her.
In "Knocked Upside Down" from The Joker: The Man Who Stopped Laughing #4 (by Matthew Rosenberg, Francesco Francavilla, Tom Napolitano), the Joker is cursed by Zatanna after he makes a pass at her and ends up becoming pregnant with his own child.
Narratively, he got a permanent smile scarred into his face just by being born, which is tragic. But cinematically, the idea came from the 1928 silent movie that inspired the character's creation in the first place, The Man Who Laughs (the link to The Batman Who Laughs is clear).
Arthur is next seen exiting Sophie's apartment but it's never made clear whether he murdered Sophie or not. A contextual clue that maybe he didn't is later, Fleck didn't murder Gary (Leigh Gill) because the little person was the only clown at his workplace who was nice to him.
Director Todd Phillips considered the character's body size as one way to represent his vulnerability. It is important to emphasize that unlike most people who diet to lose weight or who develop an eating disorder, Phoenix's weight loss was intended for the short amount of time it took to shoot the movie.
Batman and Joker definitely aren't brothers in the comics. While the Joker movie plays very deeply with this idea, it still maintains the same stance. In the books, Joker's origin story has been a mystery. Readers have come across multiple different ones over the years.
Starting with 'Joker's Five Way Revenge,' the Joker went from being either a dark, foreboding murderer or a goofy prankster to a villain who split the difference with threatening, thematic crimes that confounded Batman's status as the world's greatest detective while also standing in stark contrast to the Dark Knight's ...
Arthur was brought up by a foster mother and did not have a father figure. The foster mother herself describes him growing up as a fun and kind boy, but later he was abused and beaten by his adoptive parents and even had a severe head injury.
One of the mysteries in director Todd Phillips' Joker is whether Arthur Fleck really is the son of Thomas Wayne as his mother Penny Fleck claimed. Both Wayne himself and Alfred Pennyworth said it was untrue and that Penny was delusional, hence her eventual stay in Arkham Asylum.
Following her introduction to the comics in 1999, Harley Quinn was depicted as the sidekick and lover of the Joker as well as the criminal associate and best friend of Poison Ivy.
However, given her role in Arthur's descent into complete insanity, there's one certain thing about Penny Fleck: she is partially responsible for making her son to become the person he is now.
Fans have speculated that the baby clothes implied that the Joker may have murdered babies off-screen. Fortunately, the answer is less gruesome than that. On Twitter, Ayer revealed that they're more of a reference to Harley Quinn, more specifically, Harley's desire to have a family with the Joker.
That mystery is quickly solved as The Joker gives "birth" by puking up brown goo in the next few panels of the comic. The sludge then forms into a young boy who looks very much like his father. One panel shows both The Joker and the newborn staring at each other, thinking "he's so handsome" in a shared thought bubble.
The shot actually comes from Harley Quinn who, in Batman #99, showed that she was willing to put the Joker down -- permanently. Her shot doesn't kill Joker, but it does take out his right eye, and it's no accident.
Similar to Jared Leto's Joker approach, Phoenix transformed physically and mentally with his method madness — losing 52 pounds for the role! "It turns out, that affects your psychology. You start to go mad when you lose that amount of weight in that amount of time," the actor said at the Venice Film Festival.
