In terms of biology, two instincts describe good and evil. It is not a metaphysical thing. Even violence is not inherently evil. How can good thrive? Humans must choose.
I have written a few books on the subject of human survival and development into the future, but the main two books are "Genetics For A New Human Ecology" that I published some time ago and the book I am now working on, "Strategy For A New Human Ecology". This discussion would be part of the Strategy book. There is a lot more to the Strategy book, but the question of good and evil is one of the more important of the big questions that need to be answered both for human survival as well as for personal understanding. We can only deal well with what we personally understand. A better understanding of good and evil strengthens us.
Most people have a more philosophical or humanist view about how can humans achieve their aspirations of moral development. Since the beginnings of civilization, thinkers have expressed the drive to become more than animals, more than "man in nature" which could be a harsh and brutish existence. The two problems are related, and both must be solved for either problem to be solved. These could just as well be expressed as goals.
In terms of ecology, a problem is that humans are trying to survive the largest change in ecology that any animal has tried. The transition from the tribal world to civilization is all encompassing, and its execution is based on human behavioral adaptability. It seems like everything in the ecology is changing, yet we are mostly stuck with the potentials and limitations of our instincts. It is hard to adapt to and not necessarily comfortable, because while we have a good start, we have a ways to go to really be comfortably adapted to the new ecology. We have most of the strategies we need available, but not ll of them. As a biologist, survival is the problem I work to solve and I look at good and evil that way.
As a biologist, the problem and goal is all about survival, adaptation, and development, in far more than the individual sense. That is what is considered good there and if you disagree with that, you are not likely to agree with the rest of this. The solution to this requires not just discussion of strategy but also about instincts, which are a slippery subject at best, partly because we are culturally inhibited from looking at them. Objectively, we know that instincts do exist, are strong, and important, but subjectively we do not pay attention to them. Sometimes they provide potentials such as innate knowledge, but they also provide limitations within which we operate. It is harder to learn what we do not have much instinctual basis for. They are important to take into account whenever considering human strategies, especially competition and cooperation.
When considering strategies, a scary thing I see, is how much we have forgotten. Richard Dawkins talked about memes, contagious ideas that compete for survival like genes. Just as humanity needs to direct our genetics to our benefit, we are going to consciously need to husband our memes and beliefs. We need a description of the memes we have to work with and their consequences. Memes, including all facts and beliefs, have been called "exogenetic knowledge" as opposed to genetic information in genes.
One other question, directed at the reader, will be how can this answer matter? If you are reading this, you probably already know about the problem of good and evil. You know about discipline and self development. You know the value of honesty and the need for it. You also know that most others do not see it as you do. Most people have very short narrow views. The more you learn, the further behind you leave them. How can they be woken up? How can humanity be convinced to choose their destiny instead of constantly just reacting to what happens to them? Well, there may be a certain opening for that and just as importantly, it may be easy to take advantage of. I will get to that.
Keep in mind that the battle between good and evil is not a war to be won. It is the struggles every day by all good men and women. What needs to happen is a shift in the balance to where culturally we understand what good and evil are, and why to chose one over the other. Evil will certainly persist, but it can be overshadowed by good. If it is, humanity will survive and progress. The danger of evil will not be so near, but it will still be in us.
It is hard to go against your moral instinct.
If you want to solve a problem like the quesiton of good and evil, any solid limits you can find releated to the question are helpful. There is a useful way to describe good and evil in human nature.
All biological strategies are about survival. To a biologist, morality is a survival strategy. It is the outcome of the combination of moral instincts and a moral system. It is how we choose right and wrong in terms of survival. (Our survival instinct is why we choose, but that is another, very fascinating story.) So, if it is a survival strategy, what does human evolution say about that?
Humans basically have two natures. You hear about them in the old story of the tribal elder telling the young buck that he has two wolves inside him, one good, one evil, fighting for control. An old story it may be, but it carries valuably truth.
Humans have two basic survival strategies and corresponding instincts. The commonest survival strategy in nature is one of blind, mindless, endless competition, once referred to as "red of tooth and claw". Strategically, it might be described as "winning is the only thing". In much of human history it has meant "might makes right". A less common strategy in nature is cooperative social behavior where strategy involves survival as part of a group. Nature tends to be very selfish, and exceptions to that are unusual. These two strategies are reflected in moral systems, but more importantly in moral instincts. That makes them more than beliefs. It makes them survival options.
Violence was a pretty effective strategy in a lot of ways, but sometimes it just is not the best for survival.
About the time of Lucy (Australopithecus, say 3 million years ago), when humans started walking upright to enter the hunter-gatherer-scavenger ecology, they were poorly adapted to their new ecology. The big cats loved eating them. They were much like us, but small brained. To survive, they needed to develop better social skills of communication and cooperation. That is shown by the very rapid brain evolution that followed. They had more potential to survive by getting along and working together. It was necessary and drove human evolution. The usual blind competition and violence of nature in a tribe would endanger the whole tribe. Evolution proceeded this way until about 70,000 years ago, there was something of an evolutionary event... partly it could be said that the parietal lobe evolved or perhaps it could be said that the brain re-organized. It made humans much more efficient so that they were then dominant in the ecology. Art, tools, funerals, social customs, and other behaviors changed. We started killing all those pesky cats and everything else. Violence, the most common evolutionary competitive behavior, again became a useful strategy because we were efficient enough as a species that it no longer endangered us as a tribe. There was competition within the tribe and between tribes, though competition within the tribe was reduced. No other species competed with us and many species went extinct because of human activity. We grew and spread out very successfully. Farming and culture developed as well, leading to cities and multi-tribal civilizations. In the West, this type of violent "competitive behavior" sort of peaked with Rome. The violence was so bad that various philosophies and religions arose to try to deal with it or promote peace, such as Buddhism, Stoicism, and Christianity. So here we are now, with instincts for both cooperation and violence. We can see it every day. The future, human survival, will be decided by which instinct we choose to follow.
Take a moment here to find both of those instincts within yourself. It should not be hard. Yes, most of the time you are a very nice person, but you are not human if you do not know about your potentials for much less social behavior. You may know just how nasty your instincts can be if you are threatened. Both instincts are very trainable, and you probably have the potential to be trained as a warrior if you were raised in a warrior society. You can also be trained to be quite civil in a civil society. If you did not have both of those instincts, your ancestors would never have survived. In general, herding tribes had more potential for aggression than farming tribes. Herding developed wealth worth raiding. Farming was a competition with an impersonal nature.
Notice that violence is not inherently bad. Following the instinct for blind competition to violence, is. The problem with violence relates to mostly to two problems. Our ecology, our life support system, does not exist in nature and must be built. It can be damaged by violence, particularly war. The other problem comes down to meme competition. Creativity has been the most productive strategy for humans. You do not want people resorting to violence because it inhibits the creativity. We need to solve problems creatively, if possible. Violence though is often effective and simpler as well as having an instinctive component, so the society has to work to inhibit it. War produces nothing.
[This is missing most discussion of how to judge what is positively cooperative and comnpetitive.]
[This is missing any consideration of human predators.]