I went to see The Eviller Twin
today, a play, by Sue Cargill, about identical twins, one of whom was indeed, it turned out, rather evil. It was a comedy, of the dark variety. I found it quite enjoyable.
The identical twins were played by two women of only passing resemblance, and matching wigs, but that worked out fine. In effect, you were just asked to imagine that they were identical. I think as a writer I would have hesitated to do something like that, but as an audience member I accepted it right away.
The other question in my mind was whether the word was "eviler" or "eviller". It turns out they're both acceptable, somewhere or other, though I would always be inclined toward saying "more evil" for the comparative.
I think it's better with the double L,
since that gives eviller something in common with Hell.