Senator Stennis D-Ms was one of the senators targeted and within a year had been shot. I think this could be the reason.
That said, Kerry had knowledge of the assassinations that others in his group were advocating and failed to report it to the FBI or other law enforcement agency, which (IMHO) would make him guilty of misprision of felony. Considering that the conspiracy he failed to report took place while we were at war and was intended to give aid and comfort to the enemy (with whom members of his group had consulted before hatching the conspiracy), he would also (IMHO) be guilty of misprision of treason.