Der Beobachter (2015)

Gillian Ward believes that the secret of her secret love affair with John Burton, an ex-police officer, is in good hands with her friend Tara. When she decides for the family and ends the relationship, her husband is murdered. The traces at the crime scene point to a serial killer. Her neighbor Samson Segal, who secretly observed and meticulously documented Gillian's life, is targeted. Fearing for herself and her daughter, Gillian seeks refuge with Tara while the police search in vain for the fugitive Samson.

Unfortunately the movie Der Beobachter is not yet available on HBO Max.