What if that speech is calling for violence?
Fundamentally, government is violence. One common formulation is that the government has the monopoly on the legitimate use of violence. When a person commits a crime, the government is authorized to use violence to punish it.
Practically every government considers a threat to be a crime. A threat is speech, and the government responds with violence.
An assassination by a citizen is not a legitimate use of violence, by that definition, but we've shown that there is such a thing as a legitimate use of violence in response to speech.
And conversely, any government action is ultimately backed by the threat of violence. Ideally, that's restricted just to acts generally considered abhorrent, but in a democracy it only takes a bare majority to legitimate government violence. Speech that influences government can be tantamount to a threat, authorizing violence for acts that you don't consider to merit it.
I'm not trying to justify the recent assassination. I'm trying to show that a simplistic "violence is never a response to speech" is not a sufficient argument. Speech and violence are not as distinct as we'd like to think. There are many ways that speech can lead very directly to violence.