Yield To The Night/Blonde Sinner (1956)

A young woman who has been abused and taken advantage of by the men in her life finally finds a man whom she feels truly loves her. When she finds out otherwise, and that he had been cheating on her, she snaps. She finds and kills her boyfriend's mistress and is subsequently sentenced to death. Partially based on the real life of Ruth Ellis, the last woman who was executed in Britain.

IMDb: https://www.imdb.com/title/tt0049019