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.
Therein lies the evil. Man is not a thing. A nuclear bomb is neither good nor evil but can be used for either. A man (or a Vulcan) is responsile. Someone like Spock who could comprehend, and could even calculate the odds, had a responsibility to decide.
I know not everyone is a Christian, but Jesus said that he would rather someone be hot or cold than luke warm. Mirror Spock was luke warm - someone who could figure out the right thing to do, but didn't bother until someone pressed him to think.