This has been my belief for a long time now. Everything he does seems calculated to me. Specifically his supposed 'feelings' for Sophie.
Remember how he was bragging to his friends about how he was gonna fuck Sophie at the beginning of the game?
That up there is the real Zack in my opinion. Everything he did in front of Sophie after this point was nothing more than a facade.
His attempts to get to know her better (watching Crystal Rangers, and taking interest in her favorite books)? Merely a means to an end.
His sad puppy dog eyes because he couldn't be with her? Nothing more than a performance meant to tug at her heart strings.
He's a sociopath playing a sociopath's game. Nothing more, nothing less.
Though, of course, Azienda might be one update away from proving me wrong...