There is an old rule in general discussion groups that, once someone makes a comparison involving Nazis or Hitler, the discussion is over and whoever mentioned the Nazis has automatically lost whatever argument was in progress. There is also a rule in groups devoted to discuss fictional works that, once someone decries certain points of a story on the grounds that in real life such points would be inappropriate or illicit or wrong, the discussion is over and whoever mentioned real life has automatically lost whatever argument was in progress.
There are no guidelines for enjoying fantasy. Think of how many fictional works starring villains or antiheroes exist out there, that are extraordinarily successful because audiences love the shocks and thrills of the story. Think of how many videogames exist where you play the role of a bad guy or dark hero who struts around browbeating or robbing or slaughtering everyone he meets -- and gets away with it as long as he defeats all opposition. It's not anyone's business to judge or revile people who like such works. The things you do for the sake of harmless fun are your business alone, and only a fanatical fundie or a deranged zealot would try to mess with you for that.
I'll say it now, loud and clear: I hope Satou and Shio make their getaway and live happily ever after, and I hope anyone who gets in their way bites the dust and goes to hell. And if anyone wants to contend that such a development would be oh-so-wrong in real life, all I can say is: sorry, mate, you have just automatically lost the argument. There.