Before She Knew Him by Peter Swanson (2019)
Artist Henrietta "Hen" Mazur is convinced her next-door neighbor Matthew Dolamore is a killer. While at Matthew's home for dinner one night, Hen sees a fencing trophy that she believes belonged to murder victim Dustin Miller, who lived down the street from Hen when she lived in Cambridge. Hen isn't sure what to do because she has bipolar disorder and is doing well now, but while in college she had an episode and was arrested for attacking another student because Hen believed the student was a murderer. Hen feels the police won't believe her now, but when Hen begins to follow Matthew, she becomes a witness to his violence and her and Matthew's lives become forever intertwined.