Valdemort killed Harry's parents.
Why? We don't know.
Why were Harry's parents good, and Valdemort evil? Because they were Harry's parents.
Read the books. Voldemort is the most fiendishly evil & stereotypical villians around, despite what you hear from others who haven't read the books themselves. He kills & tortures everyone in his wake. He even -looks- evil... white skin, black attire, red eyes, almost no nose.
Extremely clear distinction. As opposed to, say, the ending of LotR. We'll see about the ending to HP though.