Nature isn't black and white, so why do we think that way about morality?
Christianity has implanted in us the idea of moral absolutes: rigid rules that are fixed for eternity. Moral absolutes require the use of abstract reasoning, but our minds are best suited for practical reasoning (i.e. real life problem solving) and very poorly suited for abstract reasoning (i.e. discovering universals) without repeated training. The mind uses mental shortcuts to make assumptions, based on prior experience, about causality even where an inference about causality cannot be made. Furthermore, not everyone has the same use of reasoning. Some people have a better grasp of reasoning than others, and must therefore be held to a higher moral standard by virtue of being more rationally inclined. You would not expect a child to have the same physical abilities as an adult, and by extension you wouldn't expect them to be equal in mental abilities either. The circumstances of our lives is another factor that adds or detracts from our ability to reason. Deficiencies in food, water and security overloads the mind with stress and inescapably leads to deficiencies in the sort of abstract reasoning required to make moral choices; a reasoning wholly detached from the world isn't possible when you're constantly on the brink of death.
Nature fixes limits to species through predation, resource constraints and disease. In nature there are no one size fits all solutions to these problems. A trait that adapts a species to one environment may produce the opposite effect of making them maladapted to another environment. In our social environments, a behavior that adapts us to one social condition may lead to maladaption in another social condition. The contrast between family life and interactions with strangers is one instance. Finding a mate in r-selected populations compared to finding a mate in K-selected populations is another such instance. The real world is filled with nuance, variation, and elasticity and thus our choices must be nuanced, varied, and elastic. The pie in the sky ethics of Jesus are physically and mentally impossible, so it is no wonder christians live in hypocrisy. You can't have property rights, and therefore civilization, without retaliatory violence. An 'eye for an eye' not 'turning the other cheek' makes civilization possible. Giving away your possessions to the poor is usually the worst way to allocate resources and alleviate poverty. A society that followed this dictum would soon find itself completely impoverished. Generosity between family members and reciprocity between strangers and friends, even the poor, is the only way a society can flourish. Christianity is to ethics what communism is to politics. The communistic teachings of Jesus are impracticable and wholly divorced from reality.