Mankind may not be getting better because, according to Richard Dawkins in River Out of Eden:
The primacy of family ties in all human societies and the consequent appeal of nepotism and inheritance.
The limited scope of communal sharing in human groups, the core common ethos of reciprocity, and the resulting phenomena of social loafing and the collapse of contributions to the public good when reciprocity cannot be implemented.
The universality of dominance and violence cross human societies (including supposedly peaceable hunter-gatherers) and the existence of genetic and neurological mechanisms that underlie it.
The universality of ethnocentrism and other forms of group-against-group hostility across societies, and the ease with which such hostility can be aroused in people within our own society.
The partial heritability of intelligence, conscientiousness, and anti-social tendencies, implying that some degree of inequality will arise even in perfectly fair economic systems, and that we therefore face an inherent trade-off between equality and freedom,
The prevalence of defense mechanisms, self-serving biases, and cognitive dissonance reduction, by which people deceive themselves about their autonomy, wisdom and integrity.
The biases of the human moral sense, including a preference for kin and friends, a susceptibility to a taboo mentality, and a tendency to confuse morality with conformity, rank, cleanliness, and beauty,