You don’t have a first amendment right to threaten people with violence. Threatening people with violence is actually a crime, and people can and do go to jail for it every day.
I know, I know, your website didn’t quite cross the line into threatening Hillary Clinton with violence. And you weren’t charged with a crime, you just got a stern talking to by two security guys who realized after they met you that you were just a parlor pink spouting off, not a dangerous nut.
But you still can’t threaten people with violence, even if they’re really evil people like Hillary Clinton. As for whether you could be arrested just for wanting to kill Hillary Clinton, well, it would depend. But if you posted on your website that you wanted to kill Hillary Clinton, and the cops thought you were serious, you bet your ass they could arrest you. It’s not a crime to wish that someone were dead, but it can be a crime to say or write that you want to kill them, or want others to kill them.
If you wanted to kill Hillary Clinton but kept it to yourself and didn’t take any affirmative steps to advance the murder (like buying a gun, figuring out her schedule, and so forth), then you haven’t committed a crime. But attempted murder is more than just pulling the trigger but missing. Simply planning to commit murder is a crime. As it should be. There is no first amendment issue involved.
You can go on and on about how you hate Hillary Clinton, how she had Vince Foster murdered, and so forth. But maybe you can see that there’s a fine line between predicting someone would get shot, and advocating that someone should get shot. Threatening people sometimes try to create plausible deniability. Yeah, it makes no sense for someone who really wanted to murder Hillary Clinton to post about it on LiveJournal. But surely you can see that someone who really planned on murdering Hillary Clinton probably is fucking nuts in the first place, and so probably doesn’t have the smoothest murder plan down. To the extent that such crimes are prevented they are prevented because the criminals are stupid and crazy, and stupidity and craziness tends to interfere with their master criminal plans.
So those homeland security guys visited you to see if you’re really a psycho, and to let you know that if they ever find out you really do plan to kill Hillary Clinton then you’ll be in big, big trouble.