I think the intermediate value theorem covers this. Meaning if a function has positive and negative values (good and evil) and it is continuous (I would assume a "vague boundary" or "grey area" or "goodness spectrum" to be continuous) then there must be at least one zero value. That zero value is the boundary.
It would indeed cover this if goodness spectrum was a regular function, not a set-valued map. Unfortunately, the same thoughts and actions can correspond to different shades of good and evil, even in the mind of the same person, let alone of different people. Often at the same time, too.
Here's the new thread for posting quotes, with the usual rules: