Remember that Kennedy could have been forgiven his drunken lust-fueled ride off the bridge and into the water. It's ghastly to cause someone to die due to negligence, which that would have been, had Mary Jo drowned or been killed by the crash, but a lifetime of good deeds may have enabled forgiveness by the general public. She did not drown or die in the crash, however.
Unforgivable is leaving an innocent woman to axphyxiate while he slept off the booze and hoped that the call to his family lawyer would have "cleaned up the mess" while leaving him out of it, to preserve his political career.
Like Charles Manson, it doesn't really matter what else he did for the rest of his life. Murder is murder.