One of the major themes in Lord of the Flies is good versus evil. Can characters or people be all good or all evil?
From the wording of the question, I would take this to mean "person" and not character. I don't believe that a person can be all good or evil. Humans are not that simple, they are a complex compilation of genetics, environment, society and culture.
Characters in a story can be either good or evil however, even when they remain flat characters. A character which is interesting to read about, like a real person, would have a combination of both good and bad characteristics.
The Lord of the Flies does not contain any one character who is only good or evil. Even Jack, who can be cast into the role is not purely evil as you could site the circumstances he finds himself in to have transformed him. Before the island, he was head choir boy and I doubt he would've been considered evil.
Each character in the story is to represent a type or an idealized version of one attribute. Piggy clearly represents intellect whereas Jack would represent brutish force. But evil or pure goodness does not exist.
This is a good topic for discussion; I've opened a discussion topic for it here. Come in and discuss this idea!