Mr. MacPherson suspects, but cannot prove, that Duddy made the prank call that ended in the death of his wife. He suspects that this is in retaliation for the insult that MacPherson made against Duddy's father, and this is partially supported by the text; it is clear that Duddy values his family above many other things and viewed the insult as far worse than its face value.
"Kravitz! Put out that cigarette immediately."
"My father is aware that I smoke, sir."
"Then he's not fit to bring up a boy."
"He's my father, sir."
"Would you like to stay on in this school, Kravitz?"
"Yes, sir. But he's my father, sir."
(Richler, The Apprenticeship of Duddy Kravitz, Google Books)
However, much of the friction between them comes from Duddy's own actions; he, like the other boys, takes pleasure in tormenting his teachers and seeing how much he can get away with. If he made the call, he never intended it to result in a death; it was purely for amusement. However, actions have consequences; Duddy thought that he was punishing MacPherson for the insult, but in fact he was responsible for a lot more, including MacPherson's return to alcoholism. Looking at the whole picture, including the insult that spurred the phone call, there is no moral justification for the thoughtless act.