While that's true, you could argue the "people are more than they seem" point to where Voldemort turns out to be good. Admittedly, that point is a little extreme, but my personal feeling is that the "first instinct" idea will actually end up winding back to Draco. His hesitance to kill Dumbledore did more to humanize him than perhaps anything in the series, and I feel that Draco may ultimately be held up as the ideal example here (especially given that there seems to be a certain innocence attached to "non-adults"). In any case, the pity Harry felt for Malfoy at the end kind of leaves a vacuum as far as an "immediate" villain goes, and Snape is the obvious choice to fill it. While this could be more misdirection on Rowling's part, there's as much or more to suggest Snape is evil as he is good.
