There is this philosophical question of how to "ground" ethics and morality, without God.
I think the answer is: ethics and morality are grounded in whatever a society happens to think about them, and they can vary from one society to the next. And societies are not always in complete agreement about what is good and evil, making some actions morally grey within that society.
Isn't this circular? I mean, it seems like a society could say "we think X is good because we think X is good, and ethics are grounded in what we happen to think", and how could anyone convince it otherwise?
The answer is: if a society really did think that X was good because it thought X was good, then it really would be good in that society, and it really would be hard to convince that society otherwise.
However, this does not mean that all societies will employ this logic in coming to decisions about what to consider good or evil. Most societies will probably employ logic like: X is good because it promotes well-being, and Y is bad because it causes suffering.
When trying to convince a society that something is good or bad, there is no "logical foundation" one can turn to. I think it's a messy matter of simply convincing lots of people using the techniques of convincing people of things.
So.. do I think people are necessarily evil if they do things that their society thinks are evil? No. I don't think people or actions are evil in and of themselves. Societies can think things are evil, and if they do, then those things are thought of as evil by those societies. So if someone does something that their society thinks is evil, then I think that person will be thought of as evil by their society.
Do I personally think they are evil? I might. If I did, I feel like all that would reveal is that I personally thought they were evil.