Mean-spiritedness appears to not be a hereditary trait. We are speaking about people that are sane, have some degree of rationality and by most measures could be classified as normal. Something happens to people that makes them mean and usually angry. The Great Depression left as many emotional scars as it did physical ones. Many never recovered.
Neurotics can be perfectly sane.
Neurosis can cause great anger and bitterness when the real world does not match up to the neurotic one. And it never matches up. Lack of love and emotion. If we humans do not get our emotional dose of love we end up not being able to love. This has been shown by research in child development.