Evil can only exist in self-conscious beings which means, as far as we know, humans or angels. Only if you have the possibility of being consciously good can you be actively bad. Evil in the animal world doesn't exist. It's true that you can have a 'bad' animal but that is likely to have a physical or environmental cause. What you might describe as spiritual evil only exists in self-conscious beings.
When God created the world, he saw that it was good. This means it was according to his plan and everything followed its true nature without deviation. Evil, when it came, did not arise through ignorance but through the will. This is the teaching in the book of Genesis and it makes sense. There is an order in creation and to observe this order is to live in harmony with the world and one's own true self. The order is based on the reality of God and includes a natural hierarchy amongst created beings. But free will introduces the possibility that beings can rebel against this order to advance their own agendas. There are several ways in which they can do this but two of the most common are by denying God and by seeking to undermine his order.
I believe that a major cause of evil in the human kingdom is resentment. It's not the only cause. Love of power is another, but resentment is supposed to have been behind Satan's rebellion against God. Pride was involved too and there is certainly a link between these vices, but Satan resented the fact that he was not equal to God and this was the cause of his fall. He rebelled against order because of resentment. This was the first act of evil and it carries on to this day. You may say that this is all a myth but it is a myth, like all proper myths, that describes a truth and the literal reality of it is not important in that respect.
Resentment is a major cause of evil in the world today. In fact, I would say it is the major cause. The serpent incited resentment in Eve, resentment against God and against Adam and against the natural hierarchy that did not place her at the top. The result of that we know. He is doing a similar thing now. He fans the flames of discontent and encourages those who are deficient in positive goodness to rebel.
Evil comes when we deny reality. Reality is God and the natural order of creation. When we deny this, we have given in to evil even though we may find ways to justify our actions. But if we look within our hearts and examine our motives for doing whatever we do, we often find that, while we may claim to be acting for the good, our reasons for doing so are not good. For instance, many political movements are filled with people who think they are trying to improve the world (according to their lights) but whose motivating force for this is really resentment and desire for power.
The archetypal rebellion is often thought to be that of a child against its parents. When a child reaches adolescence, it is quite natural that it seeks to assert its own individuality and break free of parental authority. But we have been duped into thinking that this inevitably means rebellion. It doesn't. Given a wise parent and a basically good-hearted child, the whole process can take place without incident, without major incident anyway. Rebellion is not an inevitable fact of life, either in the context of the parent/child relationship or the wider context of the evolving human being. Only if there is an excessive authoritarianism on the side of the parent or an excessive egotism on the side of the child will it manifest in the family realm. We know that God is a wise and loving parent so our current mass rebellion is not because of him. It can only be put down to our own egotism.