I believe his own movies spooked him! In Soylent Green, Chuck lives in a world where Hamburger Helper is actually Human Helper. In Planet of the Apes, Chuck is continually poked in the ass by damn dirty apes that make him wear a loincloth. Though his manic-depressive agent probably forced these roles upon him, the results were obvious: Chuck's world view changed from a happy place filled with adorable bunnies into a bleak society whose only savior is the gun.
Take, for example, the 1971 sci-fi flick, The Omega Man. The year is 1978, and America has been wiped out by germ warfare. Thanks to an experimental serum, Chuck is the last person alive. This in itself isn't so bad, as it allows him to steal cars, food, and a snoot-full of liquor. However, his buzz is stomped by some pasty-faced killer mutants that blame Chuck and his kind for putting them in their current pasty-faced predicament.
Chuck spends much of the film mowing down these mutants with machine guns (since the pasties consider themselves too morally superior to use any weapons other than 2x4s and the occasional rock). It's a lonely life, until Chuck discovers another survivor: a foxy soul sister with a taste for polyester and white meat. Together they search for a pasty-disease antidote while running from mutants, and celebrating their interracial love.
While Omega Man may not be the most skillfully constructed of Chuck's movies, it remains one of the best examples of what he would one day become. In scene after scene where he blows apart mutants with his precious semi-automatic, the glint in Chuck's eye transforms from fear to a glimmer of safety and, finally, happiness.