When O'Brien says to Winston that he's the last man with humanity, i believe he's saying that everybody else is broken and Winston is one of the last ones to break. He believes that the proles are the last hope for a rebellion and he decided to take it into his own hands in his own way. I think he was too hasty and didn't really think about it too much or plan anything out. He's an archetype for the spirt of freedom for the proles and believes that one day they can stand up to the party and make everyone else free.