According to your logic, as long as the intention is good (prevent Japanese imperialism), then the effect (threaten Japan's existence as an industrial state) is irrelevant. However, the actual intention of the United States was not to prevent imperialism per se but simply to prevent *Japanese* imperialism. In fact, aside from our own domination over the Philippines, the US fully supported French, British, and Dutch imperialism in the region before and after World War II. The motivation for the embargo was racism. You might say it was a "hate crime." Therefore, by your own logic, there was no justification for the American assault against Japan's existence as an industrial state.
Ted