Rosemary was not mentally disabled. She was manually disabled by a quack doctor performing mayhem with a butter knife upon her brain's frontal lobe. That's the kind of Nazi treatment that you got if you didn't please Joseph Kennedy.
That was considered cutting-edge treatment then - and no pun intended. He spared no expense to help his daughter. Plenty of people received lobotomies because highly- educated medical doctors said that it would be the best way to treat them.
Remember thalidomide? Another case where what was thought to be a “miracle cure” at the time turned out to have monstrous consequences.
If in ten years, it turns out that materials used in hip and knee replacement joints actually causes an inoperable cancer, will you interpret the family support of the people who had the operations really as a hidden plan to murder them painfully? No - because at the time, they made the best choice they could with the information they had.
Joe Kennedy was a mighty big SOB, but that is not why he or Rose had this surgery performed on their daughter.