My own opinion is that Colette's parents turned against Dr. MacDonald because they wanted him to stay with them and to be Colette's mother's doctor and for him to spend the rest of his life grieving for the victims. There was a female witness at the 1979 trial who witnessed similar conversations with the Kassabs when she was in the company of Dr. MacDonald.
I agree it was not good public relations to appear on the Dick Cavett show. It was a light entertainment show involving mainly celebrity gossip and not profound enough.
Dr. MacDonald admits it was a mistake to try to sooth the hysterical Fred Kassab by telling him one of the murderers had been bumped off. Segal had warned Dr. MacDonald early on that Fred Kassab could be dangerous. I reckon that could be where McGinniss got his crazy amphetamine psychosis theory from, in order to make his MacDonald case Fatal Vision book more sensational and sell more copies.
There is some background to this from an old Larry King TV show:
MACDONALD: ... and Alfred Kassab set up my interview on Dick Cavett. Years later, when Freddy changed his tack on this case and had become his...
KING: Obsessed with you.
MACDONALD: ... obsessed, and obsessed with publicity and being in front of the case -- and, very important you understand this, he was wined and dined very carefully by the CID agents.
KING: Why did they want you?
MACDONALD: I don't know that they wanted me, at first. I think once the investigation started, it becomes a team effort. I don't have major conspiracies in my head. I think it's...
KING: You don't?
MACDONALD: ... very simple. No. I think a bad cop made bad decisions that morning. Then they put me through the Article 32. Then I accused them on the Dick Cavett show of committing perjury under oath, which they did do.
KING: Your mistake was telling your father-in-law that you had the killers taken care of, right, to get him off your back? Is that correct?
MACDONALD: I did say that to him. What I was trying to do was give Mildred and Freddy a little closure. They had...
KING: No, I understand.
MACDONALD: ... shut up their house. They had turned off the lights, literally. They had a life-sized doll dressed in clothes from the kids. And I was trying to give them -- I said, Freddy, look, some of the Green Berets I know and I found this guy. We took care of it. And it was a really bad decision. I shouldn't have said that. But that is not what changed Alfred, Kassab.