On the surface, Melville’s Billy Budd is a simple story with a simple theme.
Conclusion
We should read Billy Budd as a simple tragedy.
Evidence
There's no textual or historical evidence that Melville intended an allegorical reading.
Evaluate
When we're picking answer choices that are principles to justify or conform to an argument, we're just prephrasing "If Premise, then Conclusion".
Goal
If there's no textual or historical evidence that the author intended an allegorical reading, then just read the book as whatever it superficially seems like.