Do you mean, saying you will kill "the president" or you will kill "George Washington"?
Saying, in 1792, you will kill "the president" means George Washington. Saying, in 1800, you will kill "the president" means John Adams.
If you say, in 1800, you will kill "George Washington," you are not talking of killing "the president."
"The president" refers to the person currently in the office, yes? "George Washington" refers to a citizen.
I think it would depend on the context (sincerity) of the statement. Communicating a threat, and all that. How would you feel if someone said they would kill you?
Just to note - it is, in general, illegal to threaten to kill anyone. Details vary from state to state, and making a threat on the life of a Federal official makes it a federal case...