A man who killed his step-granddad when he “apparently” saw the septuagenarian having sex with his mother will spend the rest of his life in prison.

Michael McDonald repeatedly bashed Brian Hurry, 78, with the metal bar of a dumbbell in June 2016, the Telegraph reported.

McDonald, 25, “lost control” when he saw his mother, Terri Hurry, 42, “apparently having sex” with her father-in-law at his home in Great Baddow, Essex, cops said.

During a five-week trial, jurors heard how McDonald lured the elderly man into a bathroom before beating him.

Brian Hurry died from head injuries.

At McDonald’s sentencing Tuesday, Chelmsford Crown Court Judge Charles Gratwicke called the murder “brutal,” according to the BBC.

“What is certain is that you, Michael McDonald, took that metal bar into the bathroom and set upon Brian Hurry, hitting him about the head with such force and strength that you smashed his skull in, driving it into the brain by an inch,” the judge said.

Terri Hurry was found guilty of perverting the course of justice and sentenced to 3 1/2 years.

McDonald’s girlfriend, Anna Blackwell, 21, also pleaded guilty to perverting the course of justice for giving a false alibi – claiming her boyfriend was with her at the time of the murder.

Blackwell was sentenced to 18 months.