Concurring with many of the other respondents I also find only Caroline to be a trustworthy adult. While we can and should question Caroline's parenting skills, Koch gives us no reason to question her trustfulness. Unlike the other adult characters (with the possible exceptions of Vera and Emanuelle) Caroline does not have a hidden agenda or alterior motives for her actions. She is well meaning and clearly ill at ease with the environment to which she has been subjected. That said, she is a weak character and dangerously acquiescent to her husband. That is most unfortunate given who Marc really is beneath the veneer.
As for the author's view of human nature Freud would feel most vindicated by the raging libidos on display at the summer house with swimming pool. Hedonism defined the outsized personalities of Ralph and Stanley, and Marc's secret emotions, passions and prejudices drove his illicit and diabolical behavior. Self control, honesty and respect were no where to be found in these flawed characters. In his subset of humanity Koch does not present any of the so-called better angels among us, and we do wonder whether the author finds any decency in this world.
My view of human nature is not as jaded or cynical as the author's. While there is (and always has been) more than enough evil to go around the human race would not have achieved what it has without compassion, decency, honesty, cooperation and loyalty.