Having the knowledge of good and evil may in itself be the source of good and evil. Animals, though they may do what humans consider horrendous things, possess no knowledge of good and evil. They, therefore, are incapable of doing good or evil. They are indifferent and function by way of instinct.
Humanity is capable of great good and great evil. The majority of human beings in this world possess a conscience. Most know good from evil. The majority chooses good; the minority that chooses evil is growing.
Knowing what the right thing to do is does not guarantee that the right thing will be done. In many instances if there are repercussions to doing right, doing right will be avoided to avoid the consequences. Sometimes if there is gain from doing the wrong thing, as long as no one is hurt, or just maybe hurt a little, or perhaps if the identity of the perpetrator cannot be discerned, the wrong thing will be chosen. Greed is an evil humanity knows well.
People will lie to protect themselves. If the blame falls on someone else, some may come forward and confess… but few, I guarantee. Is there a good lie? A lie like those that saved many Jewish people during WWII? Are those lies evil? Honesty works only when you are being honest with decent moral people.
A person who looks upon this perspective, feels this perspective, of the inherently flawed nature of human beings, can begin to grasp why the ancients struggled with it so. Humanity possesses the attributes of both beast and reason. The beast alone is innocent, and the reason is perfect. Combined they become corrupted. Evil is born. Initially, I think, religions were fabricated by this struggle (and to explain the unexplained to assuage fear). In time, however, the religions became corrupted as unscrupulous humans used them for their purposes.