In Matthew’s world, people usually grew up and lived their whole lives in one community. So the scenario you’re asking about was not likely one that would come to mind for the Gospel’s earliest readers. Matthew tells them that the prohibition on murder includes conflicts that are much more mundane. The teaching does not suggest that there shouldn’t be differences of opinion or even conflicts, but that one should not be driven by anger to speak or act in ways that are inconsistent with a life of faith. Acknowledging that one has done so seems to be an important step, even if you can’t apologize directly to the person involved.