However I am invoking Godwin's Law, in the case that a bunch of people get together and decide that the earth would be a better place by wiping out the Jews (because they thought the Jews were destroying Germany) what right would you have to tell them they're wrong?
Science that says Jews are no different from any other people, and that there is no evidence that they are destroying Germany? Hitler was a devout Christian and believed in some f'ed up prophesy and BS, so it was easy for him to believe in other ignorant and hateful things. Thus, it was easy for him to believe unsubstantiated stuff like "Jews are destroying Germany."
Also, second argument, an argument against utilitarianism:
Suppose you have an uncle who owns a business, and has many workers for him. Your uncle has developed a terrible disease which causes him much pain, yet he continues along with his job, instead of listening to doctors. Because of his condition he is very miserable, and so makes all his workers miserable as well. You know that once your uncle dies you will inherit a large fortune, as well as the company, so you could improve the workers' lives.
The most logical choice you decide, is to attempt to kill your uncle, after all, you empathize with his pain. If you were in such pain, a terminal illness at that, wouldn't you want an easy way out? Not only would you be improving your uncle's life, but all the workers', and as well as your's as per the inheritance.
The above would be completely moral by your view.
If death was the uncle's wish, then yes, it would be moral to help him do what he has asked for, since he owns his life and is the only one who can decide what to do with it. If you kill him despite him wanting to continue to work and live his life, then that's not moral, not by the bible's standards, and not by any other set of ethics (including atheist).