If someone who has no familiarity with the subject at all asks you what Watchmen is about, would you tell them "it's about one of their own betraying them and killing millions"? What would that tell your listener? What could they possibly learn from that? That is the least effective summary possible. You could say the purpose of V For Vendetta is that Creedy dies, the theme behind Saving Private Ryan is that their squad dies, the goal of Crash (2004 film) is that a white guy shot a black guy.