Nagisa's probably a psychopath. He's able to act like normal in scenarios like this because he's always been making a point of acting like normal. Whenever his demeanor changed before, he's probably doing that consciously to avoid outing himself. He knows that people would be freaked out if he never reacted to anything, and he knows that would disrupt his life, so he does his best to avoid that.
And of course psychopath ≠ evil. Even though he doesn't feel any sorts of emotion or empathy, he knows that life will be more difficult for him if he's a bad person. If people won't like him, they won't do what he says, he won't be able to get a job, and there would be a distinct possibility of someone attacking him in response. So being a good person is beneficial to him, and it'd be stupid of him to not be a good person.