I was thinking about this...
What if the cure wasn't developed but actually, just "Found"?
Assuming Leonard's Father died of Xeno-Poly-Xythemia (the disease McCoy himself had i "For the World is hollow and I have Touched the Sky") it might have been a short while prior to his discovering he had the same thing on the Enterprise. In McCoy's mind, it wouldn't have been too long after (in hindsight) and up until that point, it would have had no known cure.
Therefore, at the time he did it! it would have been to preserve his father's dignity and spare him the pain but later McCoy would have realized that he could have been saved had they kept his father on life support.