Believing the worst of human nature and motives; having a sneering disbelief in e.g. selflessness of others; SYN. misanthropic, misanthropical.

School of Greek philosophy (Cynicism), founded in Athens about 400 BC by Antisthenes, a disciple of Socrates, who advocated a stern and simple morality and a complete disregard of pleasure and comfort.
His followers, led by Diogenes, not only showed a contemptuous disregard for pleasure, but despised all human affection as a source of weakness. Their “snarling contempt” for ordinary people earned them the name of Cynic (Greek “doglike”).
Someone who is critical of the motives of others; SYN. faultfinder.