A cold-hearted psychopath who saw everything as a contest and who couldn't stand to lose, Nevile very nearly succeeded in having Audrey hanged for his aunt's murder. In fact, he was the one who'd bashed in her ladyship's skull and smothered family lawyer Mr Treves with a pillow - two more killings to add to that of young Peter James, a child shot dead by Nevile as a boy, using a bow and arrow as a boy. Thanks to Inspector Leach, Nevile's plan was scuppered and he was sent to the gallows, ridding his family of a foul canker. Here's how it all played out.
To punish her. An obsessively competitive man, Nevile felt that Audrey had publicly humiliated him when she sued for divorce on the grounds of adultery. He had been a serial philanderer throughout their marriage, but Audrey had suffered in silence until he began an affair with Kay Elliott. Finally deciding to act, Audrey stole Kay's monogrammed compact and planted it in Nevile's car as evidence of the adultery, which forced him to publicly confess and to grant her a divorce.
Nevile pretended to feel remorse for his behaviour and to want to stay friends with Audrey, when in reality he was seething with resentment and planning her downfall. He married Kay, whom he never loved, and arranged to spend their honeymoon at Gull's Point when he knew that Audrey - who had also grown up there as a ward of his lord and ladyship - would also be visiting. There, he carried out his plan to kill Lady Tressilian and to frame Audrey for her murder in an act of revenge.
Nevile's second wife Kay, somewhat incidentally, was revealed along the way to have been a con-artist who, along with her rakish paramour Louis Morel, preyed on the wealthy. She had first approached Nevile Strange at a tennis match in the South of France intending him as one of her marks, but then betrayed Louis by falling for Strange and his luxurious lifestyle and choosing to stay with him. That explains the tension between Kay and Louis when the gang spend the afternoon dancing at the Easterhead hotel where Louis is staying.
Trying to make Thomas Royde jealous, Mary later invites Louis to Gull's Point, making Nevile, Audrey and Kay's love triangle a love square. After Nevile's arrest, Kay and Louis are seen driving away together, reunited, and now - it's unclear - perhaps having inherited Nevile's money, inadvertently making him the ultimate "mark" of all.
When Nevile Strange's new valet "Mac" (real name Matthew Hutton) sized up his employer's clothing against himself, it looked as though he were plotting to usurp Nevile's position. Was he in cahoots with Kay, as it seemed when they shared a sip of whisky from the same hip flask? No. Hutton was simply comparing his and Nevile's shoesizes and more to spot a family resemblance because they were cousins. Unlike the book, in which there is no "Mac" character, and Nevile is not a blood relation of Sir Matthew Tressilian, in the TV adaptation Nevile is Sir Matthew's nephew and Matthew Hutton is Sir Matthew's illegitimate son.
Matthew (clearly named after his biological father) was the product of a liaison between Sir Matthew Tressilian and a 16-year-old scullery maid. When Lady Tressilian found out about the pregnancy, she sent the maid away and kept it as a family secret - explaining why she watched Sir Matthew drown without alerting any help, told Inspector Leach that "it's a terrible thing to be angry at the dead", and smashed her husband's framed photograph.