There is absolutely no way you can be certain of the beliefs of the whole world in response to something. It’s hard enough to even feel confident you truly know the beliefs of a single person.

That alone is true, but this imagery itself is so grotesque that we ought to assume it led to a change in the degree of motive to cause harm. If you don’t believe me, consider a thought exercise where the identity of the person involved is exchanged with someone who did end up assassinated this way. Would it be reasonable to conclude this influenced it?

That doesn't seem at all plausible to me. Further, I don't think you can point to lots of serious "behead Trump" chatter in the opposition; conversely, I can easily point you to "hang Pence from the tree outside the Capitol" postings that were not kidding.

Anyways, we probably understand each other at this point and don't need to belabor this, even if we don't agree with each other.

I'm only responding to your point, I wasn't making any comparisons. Perhaps if taken literally what you said may be correct - the problem with this kind of imagery is not its advocacy of a specific technique, but allowing people to more clearly imagine it and normalizing it in their minds.

In any case, I stand by my claim that to say you can know the full effect on all minds of a certain image is a poor argument to make, since it's literally impossible to know. I hope you'll at least recognize that kind of claim is worth avoiding, if for no other reason that it inadvertently undermines any other point you are trying to make.