By way of evidence for the idea that having a “bad side” is useful for introducing two-player games, consider the introductory sets for Warhammer 40,000 over the years:
We know that space marines are Games Workshop’s best selling product–and, unsurprisingly, they feature in every one of these. There’s an argument for simply doubling up on them, and letting new players divide the marines with a friend. After all, the numbers say that they’ll both end up wanting space marines more than they want Dark Eldar or Orks.
Yet, there’s always a bad guy from a less-well-selling line. I don’t have market data, but I have to think that that increases sales. Over time, players might decide that they want to play the space marines: they see the marines as the main characters of the setting, or like how they work on the tabletop. At the introductory stage, though, the villain has a unique appeal.
Games Workshop has its detractors, but I think it’s important to respect that they have been very successful for a very long time. They know how to make an introductory product. The fact that their introductory products always involve a bad guy strikes me as evidence that it’s a strong approach.