Many audiences, especially the western audiences have misunderstood the messages of this movie. This is not a movie about choosing tyranny for a greater cause such as peace. It is about choosing peace over revenge.
In the movie, the king of Qin was close to unify the whole country and thus end the continuous wars. The assassins all have grudge against the king. But they realized that if they kill the king, the whole nations will plunge into another endless wars. Killing the king won't give them back what they have lost. But it will surely cause more suffering to the people. So they choose to let go their revenge and even their lives for a greater cause, the peace. It is like what the nameless said in the movie, the ideal warrior will know when to lay down the sword.
In the movie, the king of Qin was close to unify the whole country and thus end the continuous wars. The assassins all have grudge against the king. But they realized that if they kill the king, the whole nations will plunge into another endless wars. Killing the king won't give them back what they have lost. But it will surely cause more suffering to the people. So they choose to let go their revenge and even their lives for a greater cause, the peace. It is like what the nameless said in the movie, the ideal warrior will know when to lay down the sword.