During the Middle Ages, it was assumed that the earth was the center of the universe. As such, the earth was a copy or mirror image of the heavenly realm and humans were seen as angels; they were created in God's image.
In those times, people felt that they were special, given the fact that God had made them Lords over His creation, and they were seen as inherently good. The notion of sin had not fully caught on yet and came with some of the radical Christian saints such as St. Jerome and in certain degrees with St. Augustine.
Humanism tried to shed some of the religious lingo, yet put humanity at the center of its philosophy. Humans were glorified: Leonardo da Vinci and other painters looked to give a more faithful representation of the “perfect” human body; Shakespeare began to delve into the intricate psychologies and the individual differences between one person and another.
Individualism was on the rise and human relationships grew in importance. At its core, humans possessed reason, something that set them apart from all other living beings on the Planet and which made them almost equal to God Himself.
This view continued even with Descartes, who claimed that animals were mere machines and did not have a soul because they lacked both language and consciousness. One of the precursors of romanticism Jean-Jacques Rousseau believed in the goodness of mankind. To the question of why some people are evil, he claimed that it was all because of society that corrupts us and turns innocent angels into manipulating impostors. If children were brought up in a less restrictive manner and did not witness and imitate the hypocrisy and vice of society, they would develop according to their natural state, and be inherently good.
Not all philosophers and thinkers agreed with this assumption though. Thomas Hobbes thought that humans needed rules and laws that would ensure harmony and respect among each other. These laws should be checked and enforced by a ruler or dictator. If mankind was left without proper government, they would turn into beasts and loot and kill each other because they are driven by egocentric impulses, looking for personal benefit and gain.
Somewhat later, Freud had also a rather pessimistic outlook on mankind. He claimed that our personality contains three parts - id, ego and superego - and that the major part, where dark and aggressive impulses reside can be found in the id, the unconscious. These are the strong driving forces that the ego and superego try to oppose to the best of their abilities. If not handled carefully, it could turn into neurosis. However, in the deeper recesses of our self, we are inherently evil and do not obey morality nor do we shy away from acts of brutality and aggression.
Throughout history, we have witnessed horrible acts committed by mankind. Science explains that a combination of genetic make-up and an aggressive environment or traumatic childhood are most likely factors that can turn people into cruel monsters or assassins. We label those cold-blooded assassins as mentally ill because any normal human being would feel horror or remorse about such acts.
Nonetheless, given specific circumstances, times of distress or war, any human being can shed their ethical standards and can turn from an angel into a beast within a matter of seconds. I believe we are born with both aspects, that we are capable of committing any atrocious acts, so it takes immense effort and skill to keep the beast within us deep asleep.