What was his motive for killing his own son and mistress, Steve?
Not sure if I ever heard what it was?
It was the classic love triangle. MacDowell was having an affair with Renee, who was separated from her husband, Gordon. Renee had a son with Gordon, also called Gordon, who was 9 years old. She also had 3-year-old Andrew, whose biological father was William MacDowell.
MacDowell spun Renee a story that they would start a new life together on Shetland, and that he had a job with the oil company Texaco and a new house there waiting for them to move in to. The week before they were due to fly out he said he wanted to take a short break with just the three of them. Renee dropped Gordon off at her husband's house then continued driving to a lay by off the A9 with Andrew to meet Bill. A witness, Catherine Johnstone, who lived across the road, heard a bloodcurdling scream as he killed his mistress, then his own son, Andrew. He then set Renee's blue BMW ablaze, put the victims in the boot hatch of his Volvo and proceeded to bury the bodies, which have never been found.
I think the motive must have been to conceal the evidence of the affair from his wife, Rosemary, Andrew being collateral damage in the process.