While Data takes the point of being emotionless, he does show emotion at times. I'll never forget him in Gambit Part I&II, when he was in command and was being challenged by Worf in front of the crew. You could see the restrained anger in Data's demeanor, regarding Worf's behavior. If he was truly emotionless, he's be very plain and matter of fact, without any other expression. This showed itself again in another episode, when Data was in command and had some extra as his first officer (can't remember the episode name off hand), who clearly demonstrated prejudice against Data, claiming he was making decisions without regard for people's lives. Data appeared visibly angry with him... and at first I wasn't sure if he was exercising "anger" as a means to an end without feeling it, thinking a human being who is stubborn needs to be shown anger to gain awareness. But when the XO was dismissed, it looked like Data was rather satisfied for having unleashed his harsh tone.
With Fajo, it seemed like Data had an emotional moment there as well, though very slight. Did the weapon have stun capability? Could he use a lower setting to maim him and allow justice to be dispensed according to Federation law? If not, why didn't Data merely throw himself at Fajo and knock him unconscious? I believe there was an underlying emotional response. Fajo was goading Data on... about how he can't feel rage or anger. And in essence, Fajo's experiment worked--he got Data to do what he wasn't supposed to--attempt to kill an unarmed being.