It's a function of stereotyping and shorthand. Either you have a tale "against evil" or you have a tale of "finding evil." Rarely is there time for both. if the former, the antagonist must be demonstrated early and often. If the latter, the "what is evil" question is the structure of the film. Consider THE USUAL SUSPECTS.

In your mind, is this shorthand a function of audience expectation or something else? Also, under what circumstances would that shorthand change?

When it's broken down like that, I see what you mean about the two kinds of evil tales.

It's not that she didn't feel emotion. It's not that she didn't have empathy. It's just that it didn't affect her. Her emotions were primarily about her and her empathy was in terms of her. Stephen King described a sociopath in It in a very memorable way; there's a kid that just doesn't believe anyone else in the world is real. She was a lot like that. It's not that we were all there for her entertainment, it's that we just didn't count.

That makes sense in regard to this article. I've been wondering why the killer would bother to have a partner, since it seemed like his sexual needs were met and fed by his "activities" and being a known homosexual with a partner doesn't seem like it would throw people off of thinking he was out seducing marines, especially given the time period.