Joyful and willing killing is always an evil act.
So Rorschach and Comedian are by their very nature evil.
The Comedian is Chaotic Evil - although he fights for the armed forces, he doesn't do so for any reason other then the fact that it gives him an opportunity to hurt people. He doesn't do it for his country, the betterment of mankind, or even his own betterment. He just thinks hurting people is funny.
Rorschach is Lawful Evil. Being lawful doesn't necisarily mean that you respect the laws of the land you're in (although it can mean that), it more means that there is a structure to your thought process. You have a code to determine your actions. Rorschach kills killers. Brutally. He'll maim and injure and terrorize people who interfere with his ability to dole out what he sees as just reward for preying on innocent people. Some might consider him Good, because he only kills "bad guys", but the fact that he relishes in the injury and killing of these people, his brutality and lack of any remorse.. He's evil. If you can call him good despite his evil actions, I can call him evil despite his good intentions.
Nite Owl, both of them, are probably Lawful Good. They want to help people, They have a strong code of ethics and morals. Nite Owl I was a good police officer, and Night Owl II wanted to be a police officer. In the end, Nite Owl I could grudgingly accept Ozymandias's chaotic evil act for the greater good, because, in the end, his code of ethics was largely untouched. There were still bad guys out there to fight, good people out there to protect, and lawful people out there willing to help protect the people.
Silk Spectre, I and II are probably Neutral Good... I don't really want to give too much thought to it atm... In short, they don't have the same zeal that the Nite Owls have. They generally want to help people, but they don't have the drive pushing them to right not only the evil but the unjust.
Dr Manhattan is True Neutral. That's the easiest one, because he is largely unattached to anything in the world. He has some Good moments, but hardly slips at all into chaos or lawfulness, and I think his only evil act is his final endorsement of Ozymandias's plot.. Not enough to shift him into Evil territory.
and finally, Ozymandias is, I would wager, Lawful Evil. I will probably stand to this firmly, so I may not post in this thread again (even though it has been fun thinking about all of this stuff). My barometer for a person's evil nature is.. if they can look in the mirror in the morning, and say with a smile "I killed a million people today, and I'm ok with that", then they're evil. I truly get the impression Veidt could do it. He sees very little value in a single human life, and for me, that's all it comes down to - how do you value others. If you would vaporize them, even to save them from vaporization, then you're evil. It may be a necessary evil, but if you do it and feel no remorse, you're evil.
As a final note, D&D doesn't even use all of these alignments anymore because they're too confusing. I mean, look at this thread. Noone can agree on anything (and there's nothing innately wrong with that). D&D's 4th edition (let's NOT turn it into another one of THOSE threads please), uses Good, Neutral, Evil, Lawful Good, and Chaotic Evil as the only alignments. They paint broader strokes, but it's easier to rationalize. Using those alignments we get:
Comedian - Chaotic Evil
Rorschach - Evil
Night Owl I and II - Lawful Good
Silk Spectre I and II - Good
Dr Manhattan - Neutral
Ozymandias - Evil
Still room for discussion there, but it requires a lot less thought to place them.