What does king mean when he says shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will?

When a good hearted person does not completely understand, it is even worse than an evil person completely misunderstanding. 

If people of ill will misunderstand something, they might act in a very bad way. The king is emphasizing how frustrating shallow understanding of good hearted people is by comparing them to ill willed people.  

Hope this helps! :)