Are humans basically good or evil? The question might be unsound, but an evolutionary view of the human condition needs a coherent story about the moral character of human nature. Why? Because people mean something when they ask this question, and when they encounter evolution-based narratives of the human condition, they often find those visions of human nature to be morally unacceptable. For example, a critic of The Better Angels of Our Nature said that if human nature truly were as the author described, then humans would be depraved.
See Human Nature for a summary of what we believe human nature is like, and Human Origins and Original Sin for how we got that way. Humans are cooperative and competitive, peace-loving and violent, friendly and suspicious, and all for reasons that make sense from an evolutionary perspective. Human behavior can be understood as Adaptive Behavior, but it is common for the connection to reproductive success to be somewhat subtle. This is partly because complex and overtly pointless behavior (like a symphony orchestra) is a hard-to-fake signal of individual fitness in competition for mates and social status.
Is this evolved nature good or bad? See Evolutionary Ethics for a more in-depth analysis, but the evolutionary perspective is that humans have evolved to be both cooperative and competitive, both generous and self-serving. These forces must remain in balance for a society to function, but people need little encouragement to watch out for their own interests, so most moral and legal guidance aims at promoting beneficial cooperation. Furthermore, our moral senses are the product of evolution, a sort of rough-and-ready summary of “what works” in relationships between individuals and also between individual interests and the group interest.
Any story about humans being inherently good or evil is a myth. That means we can't say whether the story is true or not, but it also means that we think these stories are very important. Perhaps our legends of the Golden Age waft up from our Collective Unconscious as memories of the Dreamtime, when we were all hunter-gatherers (see Human Origins and Original Sin). Almost all religions agree that we are both good and bad, and have many stories explaining why, such as the Fall of man, Pandora's box, the Apple of Discord, and so on.
With the lens of Cultural Evolution we can see particular myths, such as the Christian myth of Total Depravity, as being both adaptive for the religion (by encouraging converts who want God's help in being a better person), and also adaptive for groups that adopt that religion (by reminding members that they have to constantly work at cooperating better.)
We wouldn't say that humans are depraved, but we rather like saying that we are “weak in every part”. It nicely captures the truth that we are only strong when we work together, and summarizes a great many scientific findings as well. In particular, our Positive Illusions: we are not as smart or as virtuous as we think we are. Our socially approved stories of how morality works are incomplete. We don't work either the way it subjectively seems that we do, or how we are taught we should behave (and strive to appear to behave). This is for both implementation and adaptive reasons.
Although the evolutionary perspective is new, the resulting human failings were well known to Jesus, Buddah and the other authors of the wisdom literature. We frequently fail to follow moral rules, and often act in self-serving ways. But what would it really be like if we followed rules without considering the context or always sacrificed our own interests? We think that there is much to Aristotle's idea that moral behavior depends on finding a satisfactory Tradeoff between goals, what he called the Golden Mean. Doing so invariably depends on the specifics of the situation, and it simply would not work to always favor the other's interests over our own or those of our own group.
We are the way we are. Without humans there is no morality, so asking whether humans are moral is either meaningless or obviously true (by definition). A humanist says we must define what it means to be good, not because we are so good, but because there is no alternative. Without us there is no evil either. Morality is neither the free standing world of reasoning imagined in ethics, nor the ideological certainty of theology. It's a human achievement with ancient evolutionary roots (see Human Origins and Original Sin).
A far more meaningful question is, given human nature, what environment best reinforces the good? That is, what promotes human flourishing? Evolutionary thinking about human nature has been driven mainly by a desire to understand how we got to where we are, so it doesn't come with a pre-packaged political program. However it is safe to say that, just as evolution is anathema to religious conservatives, traditional social progressives will also find much to dislike. See Evolutionary Politics. In The Better Angels of Our Nature, Stephen Pinker argues at length that violence has declined both from prehistoric conditions and during historic times. Great change is possible in the human condition, even though human nature has likely changed little during this period.