I dunno. I feel like "I don't understand how anyone could believe X" is a much, much better position to take on issues than "I know exactly why my opponents disagree with me! It is because they are stupid and evil!" The former at least opens the possibility that your opponents believe things for good reasons that you don't understand -- which is often true!

I am imagining the following exchange:

"I don't understand how anyone could believe X!"

"Great, the first step to understanding is noticing that you don't understand. Now, let me show you why X is true..."

I suspect that most people saying the first line would not take well to hearing the second.