Tangential: those who couldn't care less would be nihilists, or sadists. "Misanthrope" is better reserved for something else. I guess it all just blends together from certain standpoints... it shouldn't.
Supertangential: Nihilists just don't believe existence is intrinsically meaningful, and sadists take pleasure in inflicting suffering. A more correct term for someone who is indifferent to the suffering they cause might be a "narcissist" or "sociopath".