I don't know that I'd phrase it so much as a free will thing.  In my view, being influenced by someone, or even doing exactly what they tell you to do, isn't really a loss of free will, but a use of it.

That said, I think the overall question is a good one.  My point of view is that everything we say and do influences those around us to one degree or another.  Sometimes its easy to predict how it will influence or affect someone, other times its nearly impossible to predict.  Because we're all being influenced by everyone else around us, it's very difficult to link any given action directly to someone else's influence.  Often, though, we can talk about increased/decreased chances of someone doing X, Y, or Z because of how we act or talk.

I should make clear that bearing responsibility for influencing someone doesn't take away any of their responsibility for their own actions.  When we get into these kinds of discussions, people often say "no one is responsible for anyone else's actions!  If you blame someone else, you're letting the guilty person off the hook!"  I disagree, though.  Person A can be fully responsible for their actions, while at the same time person B is responsible for their own actions, which can include actions which influence person A's actions.  The example I like to use is sitting in the pub with your friend.  He's obviously very, very drunk, and shouldn't be driving, but he asks you for your keys so he can borrow your car for a quick drive back to his place to get something he's forgotten.  If you give him the keys, and he drives your car into a bus and kills a bunch of people, he's responsible for the bad decision.   He's the one that's killed people, and he bears that guilt in full.  You, however, are responsible for your own action of giving him your keys when you knew he shouldn't be driving.  You bear full responsibility for that decision, which also lead, indirectly, to the deaths.  Your guilt for that bad decision doesn't lessen the guilt of your friend, but likewise his guilt for driving drunk doesn't lessen your guilt for giving your keys to someone you knew shouldn't be driving.

It's similar, in my view, when it comes to words.  If your friend is really angry at his ex-girlfriend, and says "Man, sometimes I think i should just kill her!"  It's not really responsible to say "well, she'd certainly deserve it."  You're not telling him to do it, but your tacit approval, even if you didn't mean it as such, just might be the thing that tips him over that balance point and causes him to decide he actually should do it.  If he does, he's fully responsible for killing her, but you'd be responsible for your unwise choice of words.

For these kind of things, I suppose what one would be guilty of would be poor judgment or irresponsibility.  If you don't want X to happen, and a reasonable person thinks that saying or doing Y will increase the chances of X happening, but you say or do Y anyway, you aren't guilty of causing X, but you are guilty of poor judgment, which has led to you being a contributing factor.

Good points in general, although what I am thinking of is a little more- and less- direct than that. For example, if you have a congregation of people who hang on your every word and tell that group "Somebody should kill Mr. x" and one of them does, what you did wasn't actually giving an order, but the outcome is certainly a forseeable one. You might have no control over which one pulls the trigger, but given a large enough group of impressionable enough people it becomes a certainty that one of them will, even if you later assert that you didn't mean it that way.
Because if your friends is talking about killing his ex-girlfriend, you can hopefully assume he is a rational decider on such an issue. When you start dealing with people who have a one way relationship  with you- hanging on your word when you don't really know them, the relationship dynamic changes dramatically, and in my opinion the speaker's obligations and responsibility increase, whereas a lot of peopel seem to argue the opposite, based on the number of people who didn't act extreemly in reaction to the words.

In reply to silveroak (msg #2):

Let's get even more specific.  Recently, there's been several cases of suicide after bullying/cyber-bullying.  The bullies words and actions clearly pushed these kids to the point of suicide.  What responsibility does the bullies bear for their words and actions?

While I agree that they have some responsibility, I also have to doubt that one person alone can really drive another to suicide. Certainly they can be the last straw, or the worst of the lot, but it takes teh rejection of either a community or someone for whom the person commiting suicide had strong feelings to create that kind of reaction. One on one it doesn't mater how extreem your speach/text is you are just one opinion unless they have some reason to give that opinion extra weight.

Which really makes this 2 questions, both of which relate to the orriginal- when that speach is itself a form of community leadership (whether preaching on television or creating an air of conformity at high school, or leading a political cause, tec.) and the questons of how trust relate to this issue- presumedly the predicatably unstable triggerman (inspired by but not related to recent events) has also invested their trust in someone pretty heavilly if they are willing to kill other people based on the general statements of their chosen leader.

Since we seem to be focusing on culpability, three things come to mind when trying weigh that up:

1.  what happened?  The worse the actual outcome, the more guilt someone bears.

2.  What was the intent?  If the person was actively trying to get the bad outcome, that's worse than if they were trying for something else but ended up with the bad outcome.

3.  Were they reasonable?  Would a reasonable person be expect the outcome that came about, or was it a mostly unpredictable fluke?  Even if a person didn't intend for the bad result to happen, if most reasonable people would have expected it to be the likely result of their action, then the person is at least guilty of some manner of negligence or recklessness.

Using the examples brought up:  A bully that leads to suicide would do very badly in the first category (very bad result), probably pretty badly in the second (they may not have wanted the person to commit suicide, but they clearly didn't have their best interest in mind), and moderately bad in the last (it's probably not what most people would expect, but they would expect some lesser bad outcome).

A charismatic leader who said "someone should kill X" might do very badly or not so badly in category 1 (depending on whether or not anyone actually did it), very badly in category 2 (they wanted the person killed), and probably very badly in category 3 (depending on just how devoted their followers were).