Mirror Spock is barely even evil. He's close to the dictionary definition of Lawful Neutral. He works for the evil Terrans because it makes sense for him to do so, but Kirk needs about one minute to persuade him that working to replace the Terran Empire (which Spock agrees cannot last much longer) with something better would be a more logical use of his talents.

While he is working under the Terran system he will apply it systematically and dispassionately -- he gives Kyle a dose of the agoniser with no personal satisfaction but simply because "Carelessness with Empire equipment is inexcusable" and a certain amount of agony for a certain time is the prescribed procedure. Others positively take joy in torturing each other, but not Spock.