I definitely believe that people should be more understanding of each other, and less quick to jump to insults and othering, but we know so little about this situation, to be so confident that it was caused by speech seems extreme.
I am also aware that a lot of the political violence of the last few years ended up not being motivated by the reasons one might naturally expect.
At the root I agree in principal though. It's, for example, still possible he picked a bad fight with an unstable individual in a bar last night (over something not politically related) and they followed him to the event he was speaking at to shoot him. I'm not as convinced I've seen that kind of thing happen "a lot", but it's true we don't have post validation yet.
How many long-range rifle shot assassinations do you know of that were not politically motivated? Jilted lovers and such don't do that. In context it's hard to take this assassination as anything other than politically motivated.
Kirk would seem like an ideal target as he has a high media profile and is not involved in running the government. I would guess that the aim is to promote civil war and thus provide an excuse for martial law.