If I was Obama I would be looking over my shoulder all of the time. A threat like that from a Clinton is not to be taken lightly. In my life I would take that threat so seriously it would require a preemptive strike.
Hillary should be the one concerned about death. She is the relative of a previous president seeking a dynasty. She is the carpet bagger senator from New York.
She is a lawyer and hangs her whole life on precedent. The Kennedy death is a strong precedent.