The problem here is that normal people don't threaten to kill other people. They certainly don't challenge another person's bodyguards in public.
Threats of violence are usually actionable (by arrest or civil lawsuit), no matter who you are or who you threaten.
There is certainly a case for making jokes about the death of a sitting President, but actually lashing out with a punch-line-less threat about killing ANYONE is simply not acceptable in our society.
It's not really even a freedom of speech issue. Freedom of speech would be saying something like: 'This person has committed serious crimes. They should be put on trial and executed'.
Threats, however, are a kind of attack-- a verbal attack, certainly, but one that promises, however vaguely, a physical attack to follow.
I like to be VERY liberal when it comes to freedom of speech and allow even the most extreme speech. I feel that if you can't stomach the extremes, you don't really deserve the middle ground.
This guy wasn't stating an extreme opinion, though. He was making a declaration of intent.