What makes you think that?
Kennedy's actions after the "accident" and society influences and pressures at that time. It seems he was more relaxed and at ease "after" the accident. And one should remember just how the family's wealth was "earned" (deals with the Mafia). It wouldn't be the first time a male from the Kennedy clan killed a female where sex was involved. Moreover, the family decided to "silence" Rosemary Kennedy's life via a lobotomy. Sometimes desperate people (and families) do desperate things.