To discuss the themes of evil as an outside force that corrupts people and evil as something that lies within people, meaning that corruption comes from within, one has a bounty of texts and ideas available to them.
One could talk about the themes by bringing in the German philosopher Friedrich Nietzsche. In The Genealogy of Morals, Nietzsche presents his belief that morals—right and wrong, good and evil—are the creation of the upper classes. Society’s rich and powerful fabricate notions of good and evil in order to make themselves look good and those who deviate from their model appear bad.
For Nietzsche, it seems like evil is an outside force. People are labeled evil not because they are evil but because it’s a convenient designation for those looking to hold onto their authority. Such ideas might complicate the themes. If evil itself is the product of a corrupt class, then that could lead one to wonder what evil truly is and if the binary between good and evil is as strong and stable as some tend to presume.
For a less contrary view on the themes in the question, consider conversing with Socrates. In The Republic, Plato describes a debate between Socrates, Glaucon, and Adeimantus on the subject of justice and injustice—two ideas not irrelevant to the theme of evil.
For Socrates, justice takes place within a society. It occurs when an array people have common interests. To prevent evil, or injustice, among interconnected individuals, it’s up to society to ensure that its members don’t have too much or too little, since either extreme can lead to injustice. Here, evil is an outside force; it comes from external factors.
Philosophers aside, there’s plenty of novels that take up the theme of evil. In William Golding’s novel Lord of the Flies, it’s possible to argue that evil lies within every person. Even Ralph and Piggy are, in a sense, evil, because they saw Simon’s murder and didn’t try to stop it.