The Very Edge
1962
Role: Mullen

Screen captures
Video on YouTube

Plot synopsis and review from Movie Magazine International:

Brett, then 27, played a full-fledged sexual psychopath, terrorising gorgeous Tracey Lawrence (played by Anne Heywood). She was 'happily' married to Geoffrey Lawrence (Richard Todd) until Mullen, who had been stalking her for some time, attacked her in her home while her husband fiddled helplessly with the latch key. She lost the baby she was expecting and tried hard both to cooperate with the police and to rebuild her shattered marriage.

The intriguing element about "The Very Edge", under Cyril Frankel's assured direction, was that Tracey Lawrence had more of a psychic bond with her attacker than she did with her own husband. ... She had a compassion for his illness and she was, ironically, less of a victim around Mullen than she was around Geoffrey. ... Brett was riveting as the tortured psycho...

From IMDb comment:

Jeremy Brett is scary and pitiable as a stalker who is obsessed with Anne Heywood's helpless housewife. I found his performance absolutely convincing and incredibly intense.

IMDb page