Poe's always been a sadist, but he keeps that part of himself under wraps, because finding pleasure in other people's pain is something the First Order does. Not Poe, who's one of the good guys.

And then he starts fucking Kylo, and to Poe's horror there's just something about Kylo that makes Poe cruel. And Kylo never complains; he cries sometimes (and that just spurs Poe on), but he just takes whatever Poe dishes out.