What led Hazel Stewart to become a murderer?

What really happened to Lesley Howell and Trevor Buchanan was a secret that lay hidden for nearly twenty years and it would have remained that way if the Ballymoney dentist, Colin Howell hadn't confessed to the double murders. For years they were the only ones who knew that their spouses didn't kill themselves. Nicola Weir looks back at what led this Coleraine housewife to become a murderer.