Batman without Joker, Mario without Bowser, Luke Skywalker without Darthvader -- every hero has their necessary counterpart and these vilifying villain finds pay homage to those whose attention is encased in negativity.
Granted the Joker is a psychotic villain and probably doesn't deserve much sympathy, nonetheless without him the Batman universe just wouldn't be the same. Everyone gives Batman all the praise and attention while those that enable him to be the spotlight are cast aside.
Villains are popular because they are figures that people hate while being simultaneously envious of their will to not be restrained by rules and boundaries. Villains also don't have to constantly worry about having their secret identity revealed, they embrace and revel in their diabolical schemes for world domination. Given the choice I think I would choose to be a super villain, having people constantly depend on you to save them can get so old.
73 Vilifying Villain Finds
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Gritty Villainous Visages
Villain iPad Decals
Hunkified Disney Villains
Untooning Video Game Characters