Actually, I'd say the Mirror Spock was worse than evil. He was weak. The logic that our Kirk shared with him on actively fighting the Empire was there all along, but Spock was willing to go with the flow as long as it worked for him. He didn't stop to think about it, so when the crew arrived from our universe he wasn't standing athwart history yelling, "Stop." He was as much a part of the Empire as Mirror Kirk was.
Which makes me think about whether I am doing enough to stop our own growing Evil Empire. That needs some serious pondering.
He wasn't weak, or even indecisive. He was simply waiting out the inevitable, having no certainty that anything he did would have a lasting effect. It's the same thing that paralyzes all of us to some extent.
What Kirk and his crew urged Mirror-Spock to do was to precipitate change, and to take part in it, because he could make a difference sooner. And that that logical contribution would be the ethical thing to do.
Mirror-Spock made no commitment, saying only that he would consider it. As an added inducement, Kirk offered his access to secret technology that would make Mirror-Spock more intimidating.
"Trouble rather the tiger in his lair than the sage among his books. For to you kingdoms and their armies are things mighty and enduring, but to him they are but toys of the moment, to be overturned with the flick of a finger."
― Gordon R. Dickson, Tactics of Mistake
But note that nothing is said in this as to what such a scholar's motivations might be.