The words "the root of man's torture by men" develop the

The words "the root of man's torture by men" develop the excerpt's theme by reinforcing individualism since the individual "man" is oppressed by the collective "men."
What the excerpt is trying to tell us is that we should value our individualism and not be afraid of being solitary from time to time. It can often be better than to be stifled by a throng of other people who want to oppress our individuality. 

The words "the root of man's torture by men" develop the excerpt's theme by reinforcing individualism since the individual "man" is oppressed by the collective "men."

Hope I helped :)