Shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will.

what is the author’s purpose for including this sentence?

The above quoted extract is the words of Dr. Martin Luther King.

Explanation:

This statement was made by him in his letter from a "Birmingham Jail". It was written as response, when he was condemned by "8 white clergymen". These 8 clergymen criticised his methods as untimely. Here King says that people who simply agree with our cause superficially, tend to do more harm than who blatantly oppose it. There is a simple reason behind this. Shallow understanding of anything leads to complacency, and this attitude is worse than opposition. In his words “Lukewarm acceptance is much more bewildering than outright rejection”.

a goverment for the people by the people