But really.... there's enough real style evil in the real world. Don't need it in a make-belief world too.
My point is, we already have terrible, dark evil in this game--pirating. A player character who slaughters thousands of people to gain wealth is as evil as they come, no less than someone who murders a child.
What matters is that the evil works in the context of what the game is exploring. This is a game about pirating, so pirating evil works. CK2 is a game about manipulation and murder in medieval courts, so associated kinds of evil including killing children work there, while they would be extremely out of place here.
In other words, the kind of evil that works in a game is not about how evil the action is, but how it fits into what the specific game is exploring.