I can't speak for any of the authors of these fics, but I think, to a certain degree, dark!Wilson is all about fantasy and wish fulfillment. In a lot of cases, I don't think it's about throwing abuse in there (and trivializing it) for the sake of creating a plot device to get a pairing to date; what's the driving force, to my mind anyway, is the abuse itself and the desire to see it inflicted on the characters.

It's not particularly in character, but then, I don't believe that the goal with a lot of these fics is to write the most in-character piece you've ever come across. But I could be completely wrong about that. *shrugs*