What about The American Revolution? or unions/strikes? There are times in history where speech lead people to push through those tipping points, and sometimes they resulted in violent revolution and others in non-violent revolution. (and we're generally okay with the violent revolutions, so long as the 'right' side wins).

Even in your Hutus/Tutsi example, you're suggesting the Hutus used speech to persuade their people to commit violence? It's still the choice of the individuals, and eventually the group, to act violently.

Unless you're saying that with enough advertising, you remove peoples' agency. That in the face of constant advertising, individuals have less of a choice and subscribe more to group think.

Maybe freewill is an illusion and you can get people do do whatever you want with enough speech, propaganda and averts. But that's a much bigger issue of human will than speech.