Let's take your claim and apply it to ISIS... Let's say this person watched ISIS propaganda and then followed their suggestions for mass murder. Is the responsibility solely with him? Even if the creators of the propaganda publicly proclaim that it is meant to inspire murder?As I've said before, the only privilege of owning a blog is that I get to write long, involved answers to this things like this, and this answer turned into a comment too long to post. So here goes.
First off, let me get this out of the way: Of course it's possible I'm wrong. It's always possible I'm wrong, even on stuff I spent my life studying. I never claimed to be the fount of all knowledge in the universe, just to provide you with content that's worth what you pay for it.
That said, to repeat the commenter here, "if you're going to represent my view point honestly," what I'm saying is that the person ultimately, legally responsible for the shooting is the shooter. I didn't even address the topic of influence or incitement in the original post because I see it as a morass. If you want maximum liberty, it's a difficult subject and the exact point where speech becomes incitement is hard to define. Maybe it's just me. I do, however, fully acknowledge that incitement is real and unstable persons can be incited more easily than others. Defining this is where real trouble starts.
My problem is I'm a very practical person and when I see a problem, I want to fix it. Everything here is based on the question: what do you do to fix the problem? What do you do with the person or people who incited him? How do you even find them? The guy with the gun in his hand is responsible and easy to fix. You kill him. Let's say you think it was Kathy Griffin who incited him. The first question you have to ask is how can you prove, beyond a doubt, it was her and not someone else? How can you prove it wasn't a belief implanted by a teacher he had forty years ago? Or by his parents not bringing him up to respect others as real people? What level of punishment is right for the inciter? Since incitement makes her part of a capital crime, an attack like this meets the definition of terrorism, do you execute Kathy Griffin? Imprison her? What can you do to fix the problem?
Just to be clear, there is a legal incitement test from the Supreme Court, and I don't think anything I've seen out of Griffin or anyone else meets it. It requires the incitement to "imminent lawlessness", not generally raising the anger level in society. That test implies something more like urging someone to "go get 'em" in a specific time/place.
Are you old enough to remember the Son of Sam killer in the '70s? Do you remember he said he was told to kill by his neighbor's dog? So who got life in prison, the killer or the dog? Nobody put the dog down. The guy, BTW, was declared mentally competent to stand trial even though he said a demon talking through the dog made him kill. I think that's an indicator of where law places the line for responsibility.
This is the answer to your question about ISIS. You say, Let's say this person watched ISIS propaganda and then followed their suggestions for mass murder. Is the responsibility solely with him? Even if the creators of the propaganda publicly proclaim that it is meant to inspire murder? ISIS is not responsible. Did they influence him? It doesn't meet the incitement test, but let's say sure, they influenced him. So what? How many thousands of people looked at the ISIS propaganda and didn't get influenced? Again, what are you going to do about them being responsible? Go send the military to kill them all off? Already being done. Now what?
Come out and specifically say that you think incitement to violence is protected speech. Or fail to do that or you admit that by your failure to do so, admit that you are wrong, and actually IT IS POSSIBLE in some cases for someone who speaks to also bear responsibility for actions that result. No, I don't think incitement to violence is protected, I just don't know what it is. It seems to me that the threshold moves with the person hearing it as well as the situation they hear it in. The question of what constitutes hate speech and what to do about it is very hard - especially with the forces out there now saying "anything I disagree with is hate speech". Not just the Antifa a-holes, but the actual Democratic establishment. In case people forget, in 2014 the Democrat senate voted to "partially" repeal the first amendment. The move to nullify the Bill of Rights doesn't just apply to the second amendment.
A few minutes after the Republicans voted to "reduce and rename" Obamacare, a dozen Democrats came out saying the Republicans were going to kill people. Political rhetoric or hate speech designed to incite violence? What if they intended it to be (A) but it turned out to be (B)? What do you do about that? What's the fix? We're supposed to negotiate outcomes, but how do you negotiate something with a guy like the FN shooter, when top Democrats convinced him you want to kill him when you say Obamacare is terminally AFU? I personally don't think that gives us the right to go shoot those Democrats. FN shooter obviously disagreed.
For your information, I don't live in a "silly libertarian fantasy land". I'm rather perturbed with the "movement" such as it is and only use the name in the loosest possible way. Even then quite possibly the wrong way (see the paragraph up top about being wrong).
If you're going to consider what I said truthfully, you'll see that I never endorsed any prior restraint. I simply advanced the notion that one might actually sometimes bear responsibility for one's actions. For example, perhaps publicly calling for a military coup against the elected president might be something that could possibly merit legal consequences? I don't see where I said you endorsed prior restraint, but it seems to me that if you start charging people with incitement and imprisoning them that itself will restrain public speech. The act of arresting people for influencing others will be prior restraint.
Again it's all about where you draw the line and what you actually do to fix things. Let's take the one you mention about calling for a military coup, or like HuffPo yesterday, calling for Trump to be tried for treason and executed. Are these normal griping or do they need an investigation by Secret Service? Do you extend that down to Madonna dreaming about blowing up the White House, or that other idiot (Naomi Judd) at the pussy-hat rally? With the entire left wing media (redundant, I know) doing this crap 24/7, you're going to really overload the investigators.
Frankly, I don't know where you draw the line. I do believe the vitriol in all political discourse is way too overheated, but like everything else, it has been on this trajectory for a very long time and the only way it reverses course is for everyone to decide they're going too far and tame their rhetoric and that just ain't gonna happen. It is literally tearing the country apart.
Several commentators, Daniel Greenfield (Sultan Knish) and Angelo Codevilla for example, have said that we're in a cold civil war. I've said the same thing since 2010. A common interpretation of the "baseball attack" (like here) is that the war has now gone hot. I think it went hot some time ago, perhaps Berkeley but perhaps earlier with the attempts by the left to start violence at Trump rallies over a year ago. It's possible it's heating up more, but we'll only know if the tempo of operations goes up.
I've always criticized lawmakers for never asking, "and then what?" when they propose legislation. I'm just trying to ask that of myself, as I always do.
Edit 6/17 1015 EDT: I accidentally insulted Naomi Watts by using her name in place of Naomi Judd, up above. Of course, I know neither of them, I just liked Naomi Watts in the only movie I know I saw her in, the 2005 iteration of King Kong. In the weird world in the intertoobs, there's a greater chance she'll see this than things I'd say to friends. (Approximately a 1 in 27 trillion chance).
Edit to edit 1200 EDT: Reader JD(not the one with the picture) points out that the aforementioned moron is actually Ashley Judd, not Naomi. I'd also like to point out that my previous mention of the odds Naomi Watts would actually see this is an example of the axiom, "47.3% of statistics are made up on the spot".