Police first got the call from the man's wife just after 10:00 PM on Thursday. They say the woman called to report that her husband was having a psychotic episode, and was beating her grandmother with a golf club.
When officers arrived, they found the 86-year-old woman dead and the man sitting in another room of the home.
He was arrested without incident.
"At this point the male is being interviewed by detectives. They're trying to find out exactly what set him off, and we're trying to find out why this all happened," said Sgt. Jon Arnold with the Sandy Police Department.
Police believe the grandmother had just recently moved into the home with the couple and their children.
Everyone was home at the time of the incident, but the wife did have her kids run to a neighbors home for safety.