So this is the standard question of whether some speech can be worse than other speech, which is of course true, raising the question, should we legally distinguish them, which of course, we do.

I think a game may incorporate any otherwise illegal or immoral acts without itself raising any moral questions, to the extent that we don't see the actions in the game paralleling actions the player would otherwise commit IRL. Mass murder in a video game isn't seen as soothing the yearning of millions of otherwise mass murders, so there's very little moral angst. Of course it may cross some gray line which makes it hate speech and illegal to sell in certain jurisdictions, but otherwise you are free to not buy it, or organize a boycott of it, and to a great extent this is already happening and perhaps even to some extent actually reducing the commercial market for such games.

To the extent that indie or non-profit games integrate objectionable material, policing the game is no different than policing what I write in my own journal. We do certainly regulate the sale and in extraordinary circumstances even the possession of such material.

There is slavery in the Warcraft lore, the humans enslaved the Orcs. Surely such a specific example as yours would raise serious questions, but what about the Orcs?

Well, it's always about the specifics too. After all executions are also in tons of movies and books (and perhaps games), that doesn't mean it's the same everywhere.