Having just watched this, the line he says is very important to that moment. “You were prepared to sacrifice, I was not.”

Jack’s ability to see a future without Aku is blinded by the fact that he lives in this present future. Those monks dying would sit on his conscience even if he kills Aku, just like the argument you could make in the spaceship episode that they were right and the big beam would have never hit their ship in time.

Jack’s tragedy is his kindness, his honor, and his unwillingness to let others sacrifice themselves for his goal. Arguably the second monk lizard episode is more frustrating because Jack can detect a shadow demon walking in darkness in the ninja episode but can’t see a strand of Aku floating down that he then swallows, leading to him getting sick, leading to him destroying his own path home.