He was warned not to fly in those conditions. He ignored the warning. If he wanted to risk his life ... well, fine. But he was also responsible for the deaths of his wife and sister-in-law. Hubris, plain and simple.
He insisted on bringing Carolyn his wife to the wedding of his cousin Rory and they were to drop off his wife’s sister on the Vineyard then go on to Cape Cod. According to “The Kennedy Heirs” he was trying to repair the relationship with his wife.
RFK’s widow Ethel said they didn’t really have to go to the wedding. JFK Jr assured his wife this was the last time they’d do something like this, go to the Kennedy compound for a big event.
It was.
>>By that April, the two were in couples counseling, Taraborrelli explains: “Both wanted to improve their marriage. John didn’t want to be one of those Kennedy men who didn’t care how his wife feels.”
When they were invited to the wedding of John’s cousin, Rory Kennedy, the youngest daughter of Bobby and Ethel Kennedy, in July 1999, Carolyn told John she did not want to go. But several days later, she changed her mind, knowing how much it meant to John. She invited her sister, Lauren, to come along on the flight.
https://people.com/politics/jfk-jr-carolyn-bessette-marriage-struggles-kennedy-heirs/