"There is sufficient reason to suspect that Kathleen Agnes Kennedy's death was not accidental".
Let's look at the Monarchy
Kathleen(a Catholic) was married into the British monarchy:

In 1943 she came to England to work for the Red Cross and married William Cavendish, the Marquess of Hartington the heir of the 10th Duke of
Devonshire.
www.youandyesterday.co.uk...
The Marquess did his duty in WW11 and fell 4 months later to a German sniper. This left a boisterous widow in England. How did she conduct herself?

Kathleen Kennedy, known to friends as "Kick", married William John Robert Cavendish, Marquess of Hartington, a Protestant and the eldest son and
heir of the 10th Duke of Devonshire on May 6, 1944. Other than her brother Joseph P. Kennedy, Jr., no one from the Kennedy family attended the
marriage ceremony. Her husband was killed only four months later by a German sniper in World War II and his younger brother Andrew Cavendish, married
to Deborah Mitford, became heir to the dukedom.
Popular on the London social circuit and admired by many for her high spirits —
though more traditional members of British society found fault
with her boisterousness — the dashing young widow eventually became the mistress of Peter Wentworth-FitzWilliam, 8th Earl FitzWilliam.[1] The
couple planned to wed after Fitzwilliam's planned divorce.
Bold is mine. She was "Kick". Kick was wild. Kick was bold, Kick was everything that the royal family didn't need at the time. Already reeling from
the abdication of a king in Edward VIII, wedded to a commoner, let alone a Catholic:

King Edward VIII, 1894-1972
Later Duke of Windsor. King for just 11 months between January and December 1936, after he succeeded his father George V. Abdicated because he
insisted on marrying "the woman I love", Wallis Simpson, provoking the biggest royal crisis of the 20th century.
www.guardian.co.uk...
Could the monarchy take another hit so soon? Would another scandal harm the monarchy at a difficult time in the 20th century? Kick was interesting BUT
Kick was Catholic. Religious doctrine hurt her, not only in England but with her own family. More on that later.
The meat of the issue:

the dashing young widow eventually became the mistress of Peter Wentworth-FitzWilliam, 8th Earl FitzWilliam.[1] The couple planned to wed after
Fitzwilliam's planned divorce. Instead, while on a trip to visit Joseph Kennedy Sr. and gain his blessing for their relationship, Lord Fitzwilliam
and Lady Hartington died in an airplane crash over Sainte-Bauzille, Ardèche, France.
Would this lead to the monarchy to take action on this person? Kick already had a "kick" at the can, marrying a member of the royal family. He died
doing his duty. She was going to marry another person of distinction. Would the Windsors take this at par? THAT is the question. Do you think that the
royal family is willing and able to act
nefariously on an individual?