Is there such a thing?
Are good and evil defined as actions or can people themselves be good or evil?
In entirety or just for a moment in time?
I think both. A good person can make a mistake, in a moment of passion perhaps, and do something evil. It's the "thing" they did that was evil, not necessarily the person.
That good person will feel remorse for what he did, and most likely will never do it again.
But look at people like Charles Manson....he was, and still is, pure evil. If Satan is living inside of anyone, it's him! Someone that shows no remorse of the horrible things they've done to others, has no "good" in him. But a good person, can sometimes do evil things because of a situation.