In Watchmen, Adrian Veidt murders Edward Blake, convinces Jon he gave the people he loves cancer, framed Rorschach, and then managed to kill 15 million people and frame Jon for it, uniting the world in peace (movie ending). The whole point of Watchmen is to challenge philosophy and much more, including what is moral and good / evil. While Adrian killed more people than Hitler did, and in about 35 minutes time, he also saved the world from destruction.
Let's look at what happened; if Adrian did not do this, more people would have died in nuclear war. "Killing millions to save billions". No doubt that (if not for Rorschach's journal maybe) he did, in fact, save the world. Adrian's actions are a critique on Utilitarianism as well as moral relativism, and a great one. Is he truly a villain, or is he the savior of the human race?
A villain.
His means to achieve what he wants are based upon deceiving others and making the entire world live a lie. He's taking the sole responsibility for making a gigantic decision which will radically affect everyone's life, without consideration for their will or any recognition of a "say" for the world in its own supposed destiny. The inherent problem with assuming such an enormous responsibility is that since you can not know the outcomes for sure, you shouldn't assume such responsibility in the first place. We live in a life where we don't know many things for sure, to start making decisions for each other and basing them on lies is a horrible thing to do because you're taking away from the life of others. You're robbing them of the ability to live their lives knowing the "truth" and thus as well affecting the decisions they're making based upon those lies. You would've chosen to make everyone live a fantasy merely because
you thought its for their best interest. So in essence what i'm saying is that even had his plan worked, it would've still been wrong in my view.
And then of course, as you noted in the OP, the other disaster is that his plan didn't actually end up working. So it was all for nothing. At most, what it could've prevented is an immediate nuclear war, but it has failed in its primary objective, which is "unit people for ever". The irony here is that by the failure of his plan, all he did was add to the atrocities committed. He has committed the worst massacre ever, and hasn't changed anything in regard to human nature. This is in my view, like i said earlier, an inherent problem with assumed responsibility and one of the main reasons for the disasters that occur in real life, and why (among other reasons) there are so many wars and atrocities being committed around the world. Governments lie to the populations they're governing, make decisions for them, and do all they can to make sure the population is living the fantasy of their choosing. I think that some of those governments, presidents in specific or politicians in general are actually sincere, in that they think they're doing the best for everybody. Thats why i don't see what Adrian Veidt did as any different from all the other atrocities committed, only excluding ones where the intents of the person responsible are clear to not be including in anyway everybody else's best interest.
One last thing is that in his own mind, even if he thinks that he's doing the right thing, i'd expect some remorse. You don't kill 15 million people everyday. I'd expect a normal person to totally crash down after doing such thing, but since he's immensely deluded by his own ego i don't expect that, just
some remorse. I didn't detect that at all in the movie.