Barry Allen's mother, Nora Allen, was killed by Eobard Thawne, also known as the Reverse-Flash. This is the canonical explanation from the comics and is supported by the Arrowverse and other DC media adaptations. The murder was pinned on Barry's father, Henry Allen, who was wrongfully imprisoned for the crime
. In the 2023 The Flash movie, while the film does not explicitly show the killer, it follows the canonical route that the Reverse-Flash is responsible for Nora's death. The movie leaves some ambiguity but suggests that the Reverse-Flash's role as the murderer would be explored further in a sequel, which has not yet materialized
. Thus, the most widely accepted and official answer is that the Reverse-Flash killed Barry Allen's mother.
