I guess when I look at real criminals and those we truly call "evil" in real life, and then I look at these characters that have been passed off verbally as "evil" I think a puke a little bit in my mouth. For example:
I am Darth Vader, I am evil, I love flowers, and llamas, and Stacey's mom has got it going on, dude did you see her last night? Man, I wish I was that guy with her. . .FUUUUU UUUUU *stabstabstab* Dude that was fun, now lets go grab a beer. . .
^ Yeah no.
I think the unspoken aspect of evil is much better than some raging Darth Vader type figure.
Also with the harry potter villains thing, like he's not a sparkle animal that goes around killing people, his character has depth, the developmental stage is obviously there, and his creator sat down, and took time to write a story about why each character, and whatever house would be considered "evil" I never felt like (even though I greatly dislike Harry Potter) that the villains fell short. They reached their bar of "evil" without seeming completely fake. I think that might be the problem with RP characters that are "evil" people just don't take enough time to think past "I had a bad time with that girl so I killed her and now I kill you all."
Look at your life persay for instance, I'm sure we have all had a bad experience, and we aren't serial killing mass murders with a vendetta against grandma and grandpa. And if we are, they why does Earth still exist?