Kirk's opinion got him killed because some people are lunatics, not because having "mean" opinions actually justifies violence... despite what I continue to see here and on cable news...
If that's true, than you can't blame the rhetoric on the left for it either.
The truth is, a number of things played a role, to varying degrees.
1) Some people are lunatics.
2) Charlie Kirk said, during his life, some incendiary statements including, his statement - regarding school shootings - that some gun deaths are an unfortunate cost of second amendment.
3) Rhetoric from other people in the Trump administration, most especially our president, includes escalating violent imagery and words.
4) There is a concerning amount of rhetoric coming from the left as well, and that does need to be acknowledged. Personally, I do not agree with a lot of the immediate response that I have seen from my liberal friends on Facebook, as I believe strongly in honoring people in the days after their deaths, particularly when they die violently, except in the most extreme of circumstances (Bin Laden, for example).
5) Sound bytes and other propaganda tools across the internet - but particularly on 4chan and Twitter - are escalating tensions, manipulating words, and leading to a 24/7 sharing of disturbing imagery.
6) We have a mental health crisis in this country. All of the above are things that mentally ill people are susceptible to.
It's like the statement, "guns don't kill people; people kill people." On some level this is true, but guns are often the catalyst to deaths that would not otherwise occur.