I think this post starts to get to the heart of why ideas are frightening.

At first glance it seems strange to have evolved any mental system that attributes such weight to something (intellectual discussion) that has no immediate survival consequences.

However studies have shown that status (community judgments of different members value) and legitimacy (whether a person has committed an appropriate or socially taboo action) do carry with them significant effects on survival, and in severe cases can last across generations (making them worse than say, being eaten by an animal). This is because status determines who has influence (and may determine if one gets to eat or not), and legitimacy determines whether one is attacked (in a communities eyes, punished) with people being so willing to enforce these ideas that they are willing to suffer in order to maintain them.

In this sense the quote is entirely correct, thought is the most terrifying thing because thought carries with it changes in status and legitimacy rules. The examples in the quote demonstrate the power of thought, highlighting the kind of traditional social defenses thought can destroy.

An insult, is the very name we give to incidents of this fear, the more directly we concentrate on the person speaking the more obvious the association, but fundamentally when thought is most powerful it alters our status and legitimacy values, and so, regardless of how obliquely we make statements, they are always going to be frightening, and thus experienced as an insult.