Assassination upon a President is a very serious offense. How serious? It is illegal to even joke about it or to say it.
We have or hear and seen, when a person is very angry at another person, it is possible some of them might say "I am going to kill you", but you are just letting off steam, u don't really meant it or sometime a person might just to say it as a joke. But regardless of ur intention, if u say this about a President, it is a criminal offense, as serious as an actual attempt assassination, this is because, the President, as the head and leader of a nation, carry such a great importance, that u can't afford for him/her to be assassinated, so even joking about it, is legally consider as an actual threat and assassination attempt. Right now, Trump is not yet President, but those who talk about killing Trump, after he is sworn in, if they still say stuff like that, that is a criminal offense.