I also think everyone is overthinking this Draco/Snape/Villian thing. Its set up like this to show contrast. JKR has even said she intended it this way. Even though Severus is redeemed, he's been seen as the "bad guy" for so long that its a natural fit. Same with Draco...he might not kill Dumbledore, but this doesn't redeem him in the eyes of Harry and most everyone else.

This is a trivial matter.