Golding had a decided opinion that there was innate evilness in Homo sapiens--probably of a biological nature rather than theological--as anyone who has ever read Lord of the Flies can attest. In The Inheritors, Golding takes his ideas to which he gave practical demonstration in Lord of the Flies and gives them historic demonstration. A comparison and contrasting of the cultures of the Neanderthals and New People in Golding's novel can only show what Golding believed and meant to convey, that New People (Homo sapiens) possess inherent evil and Neanderthals posses inherent good (he would have been interested to know that genome research has identified Neanderthal DNA and traced its presence in some living Europeans).
Golding describes the New People as having separated themselves from the natural world, from animals, from scents, from the need to respect animal life. He also describes them as having a lust for power in all activities and all relationships, even intimate family relationships. He also describes them as being motivated by fear in addition to their need to dominate, a need demonstrated in all things, even in their religion. This is a description of what Golding considers "evil."
On the other hand, Golding describes the Neanderthals as being part of the natural world and as having a deep, inviolate respect for animal life, not even feeding off of other animals (as other animals themselves do) unless they happen across an animal slain as prey. He also describes them as having relationships based on love, pleasure and gentleness, which contrasts sharply with the New People's power- and dominance-driven relationships. Golding also suggests that fear is the most alien emotion to Neanderthals, so while they do try to rescue Nil's baby, they seem not to be motivated by fear but rather more by a need to reclaim their own. This is a description of what Golding considers "good."