Devil's advocate here, again. I don't think it's that hard to identify with Sensei. And I still don't see him as the almost infinitely bad person.
If tomorrow I, same as Sensei, wake up in another world in body of teacher, who's surrounded by pretty girls, with one almost throwing herself at me right away, others being mostly friendly, with most men conveniently missing, etc ... then "fuck yeah, I'm building a harem!" will be one of my first thoughts. For that I blame games like this.
Right from the start I could assume that I'm fucked. Me appearing in this world would be something I couldn't explain. But I'd know that the same force that brought me could remove me from it at any time, turn me inside out, or do anything beyond my imagination to me. Can you imagine knowing that and just living a normal life? Being cynical as hell would be one defense mechanism I'd definitely use. So anyone reading my thoughts would not get a pretty picture. I'd be much less ambitious than Sensei, more careful, etc. But different people, different personalities, maybe he was some 'must fuck all' playboy type in his original life, and it stuck with him. As a player, I can get over that.
With the knowledge that horrible end may be just a second away at any time, why even hold back? Other than for strategic reasons, i.e. that it may not happen that fast, and because the world otherwise functions, meaning for example that I could end up in jail, if I'd go overboard. And the more I'd discover that I can question whether anything is real or not, so in turn whether anything I do matters, the less reasons I'd have for that. So manipulating few girls by being nice to them, and without really trying to hurt them, is not that bad in this context. It would be bad IRL, because everyone knows, even without personal experience, that harems don't work very well, because people get jealous and stuff. But maybe it's a flaw that can be fixed, and where better try it than in a world that's maybe not real anyway?
I think this is important aspect. Sensei doing what he's doing in what he'd think is normal world, that would make him very bad. But he knows very well that there's nothing normal about the world he's in. It changes things. It doesn't make him good. He's pushing the limits, but given the circumstances, I can excuse that. The lack of clearly negative motivation is good enough for me.
I deliberately don't give too much importance to "happy" scenes, because I'm not sure what to think about those. Is it supposed to be real, or some nightmare, hallucinations, or whatever else could it be? The thoughts in there, are those really his? I don't know how to work with that.