There is an event in Joker, the film directed by Todd Phillips and starring Joaquin Phoenix, that was never clarified... until now. An interview with the film's director of photography revealed the truth.
SPOILER ALERT
According to Lawrence Sher, Joker did not kill Sophie, his neighbor in the building, played by Zazie Beetz. This is because the Joker only attacks those who have harmed him throughout his life.
"We wanted to make the interpretation of what is real versus what is not real part of the viewer's experience. For example, his relationship with Sophie is a fantasy for him. Some people have asked me: 'Did he kill her?' Todd [Phillips, the director of Joker] makes it clear that he didn't kill her. Arthur kills people who have harmed him in some way, and Sophie never harmed him," he stated.
The uncertainty about Sophie's fate is due to her last appearance with Joker as she pleads for him to leave the apartment. Joker looks at her and then the scene cuts to the hallway of the building. It is not shown how Sophie convinced Arthur Fleck to leave the apartment.