Ok, in answer to your title question, I'll try. ;)
1. The assertion that everything must have an opposite to exist is false. Things exist. Some are opposites. Good and evil are not opposites, nor even in the same genre of existence. Good exists in physical reality. Evil only exists in human psychological constructs. There are 'bad' things without humans (rabies, for example), but they aren't 'Evil'.
I define Evil as an action taken based on an unquestioned belief.
People can believe in God and also act in evil ways. In historical sense, this is the source of more evil than any other (so far). People who are skeptical of their own beliefs, and who use experiences or empathy to moderate their beliefs through compassion, etc., can believe in anything they want, and probably won't act in evil ways.
Bad things happen. Good things happen. Humans and other living things are physically "good" when they contribute more to their environment, community and/or future than they consume in resources.
Religion (a system of some belief or another) is a tool for community building and is often very good for the people and places where it has been implemented.
Fanatical (unquestioned) beliefs, on the other hand, are usually bad for people, their places and the future of their community.
God's existence is irrelevant to good and evil discussions in the real world, especially when people don't stay anchored in the real world when discussing their morality.
God can't tell anyone to do good or evil acts, but people often use such an imaginary belief to make decisions. Few who follow Christianity, for example, will openly discuss the construction of the modern bible books, the editing of them, and the translations and humanists that decided the words passed down from historical stories.
Even fewer people (secular or non-secular) will allow open discussion of morality outside the anthropocentric versions.
"What are people for?" Is a simple question. It is a complicated social, economic and statutory question, but failing to ask and answer it renders all other moral questions rather moot in the long term of the planet.
There's plenty of local-direct morality that can be accepted and legislated (human rights), but the right of humanity itself to destroy the planet also needs to be considered.
Whether or not God exists, humans claim to have agency and morality. War, famine, unfettered populations, and pollution prove that God has no interest in people's choices.
Advertising sure does, though, and God has typically been nothing more than the marketing department for town/tribal meetings. What the town does during the meeting is up to humans. We know this because God never shows up at the meetings, even if he is sent thousands of invitations.
