The words “should” and “must” are modal auxiliary verbs or simply modals. They provide information about the function of the main verb following it. Both “should” and “must” are similar in meaning except that “must” is a much stronger word as compared to “should.”
“Should” is the past tense of “shall.” “Should” is used to denote recommendations, advice, or to talk about what is generally right or wrong within the permissible limits of society.
“Must” is used to talk about an obligation or a necessity. It is used when people are compelled to do something.
Try re-reading a case like Kinsella with this in mind, and you'll see what I mean. Have fun!