Drow began as the evil enemy. It was a certain ranger (who shall not be named/publicised) that broke the dam wall and opened the floodgates to the point where 'good drow' are now almost their own race. The same could happen to any evil or monster race.
In a PnP game you can determine which races are playable and which are not, or at the very least limit their numbers and demand a really good backstory. In a CRPG it is not possible to do so, and in co-op games (and worse in MMOs) the number of such races employed as PCs can make them just another race.
