Bear in mind, Good and Evil are physical, albeit intangible, concepts in DnD. You can literally have places that have a higher concentration of Evil than others.

In essence, the man who would do Evil given the chance is Evil, but not because he's committing a sin. Rather, he would do Evil because he is Evil. Free will and souls make the metaphysics a whole lot more complicated, but that's the general idea. You can't say he's neutral until he commits an Evil act any more than you can say an unidentified object is electrically neutral until it repels a positive test charge.