This is what I think about the movie and be warned, there are spoilers.
Without any prior knowledge on the Watchmen, I expected another super hero movie portraying a white knight with great sense of justice and morality. Contrary, Watchmen was quite dark. Criminals exploding with guts dripping from the ceiling, arms and legs of criminals broken, and millions of people dying for the greater good. Yes, quite a dark method of justice even for the Dark Knight. However, one can say the justice in this movie has more realism as opposed to what you may see in a comic book or one can even say this is not justice at all.
Regardless, the main character Rorschach leaves his soul, his identity, behind him to pursue a life of a dark justice where he would punish criminals in the way he sees fit which can even resort to murder hence why I put great emphasis in dark. However, in the end we see a more white-Rorschach shine when he dies. Further more, the characters are developed quite well, but not to my taste of super hero qualities.
Moving into the story, Adrian, who is a member of the Watchmen, devises a plan to produce a common enemy for the world creating the theme “sacrifice for the greater good”. Sharing a common enemy for the world to hate would ultimately unite the world and humanity as a whole leading to world peace. In the eyes of Adrian, for peace, sacrifice was an essential even if it was his super hero kinsmen and even if a whole population of a city had to die. In the end, Rorschach dies refusing to keep this secret and firmly sticking to his beliefs. Believing peace should not require such manipulated sacrifices, his death whitens his character, especially for a guy who kills murdering criminals. Ultimately, this whole movie falls in to yin and yang: There’s a little bit of evil in justice and a little bit of justice in the evil.