You know, violence is one thing, war is another. Suzaku's path actually helps prevents drawn-out war. The whole cycle of violence...that's something Lelouch said he would end too. By saying Suzaku is childish for wanting to end it, and considering Suzaku's way would lead to less war casualties...what does that make Lelouch?
And the Nazis? A completely different level. The Nazis were strong, but not nearly strong enough to overwhelm their opposition. They were surrounded and outnumbered. Whereas Britainia surrounds and outnumbers Japan completely. Truth to tell, only an idiot would rebel against such a force, a backwards, nationalistic idiot or a magician.
But that's beside the point. Britainia doesn't kill without reason, like the Nazis. Nor do they round up all numbers and dispose them. Nor do they show great privilege to Britainians who get in the way of the mission, as seen with Lelouch. They follow their own martial law. Even Clovis had a reason to purge the ghettos, remember.