> Cynicism refers to a negative appraisal of human nature—a belief that self-interest is the ultimate motive guiding human behavior.

This is a poor starting point, since there are also neutral and positive appraisals of self-interest as the ultimate motive. That includes biologists from Darwin to Dawkins and economists from Smith to Hayek.

  It is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own self-interest. We address ourselves not to their humanity but to their self-love, and never talk to them of our own necessities, but of their advantages.

If you agree with Smith's famous dictum, self-interest is not by itself a negative appraisal. Most moral views can be interpreted as compatible with self-interest, if that is defined in a sufficiently long term. Can someone who has a neutral or positive appraisal of natural human self-interest in general be cynical? If not, and if human nature is actually driven by self-interest, then a non-negative attitude toward it is healthier.