Mr. MacPherson is a teacher in Duddy's high school, and is a victim of pranks by the boys. At one point, he insults Duddy's father, and Duddy takes the insult very seriously. When a prank phone call results in the death of MacPherson's sick wife, he starts drinking to excess and breaks a personal rule of his, that he will not use corporal punishment on the students. This shows how much he valued his wife and how he felt that his life no longer had meaning; he suspects that Duddy was behind the prank phone call, but has no proof.
...it all came tumbling down on Mr. MacPherson -- the drinking, the phone call, how Kravitz was master of the classroom and he was being ostracized in the Masters' Room. A quiet little chat after school was out...
(Richler, The Apprenticeship of Duddy Kravitz, Google Books)
The effect is to turn MacPherson from a target to a direct opponent; he takes pleasure in insulting Duddy and finally admonishes the boy, "You're going to go far, Kravitz." This is not meant as a compliment; MacPherson recognizes Duddy's ability to turn situations to his own advantage and is disgusted by Duddy's seemingly conscience-less attitude. While MacPherson initially believed that violence never solved anything, he came to believe that the strong preyed on the weak, and that he was the victim of unnecessary retribution by Duddy for what he saw as a harmless remark.