To be 100% fair, Japanese had rather low opinion about Americans too, especially during the beginning of the war when the Japanese achieved easy victories everywhere. Many crimes were committed on American POWs (which were also considered only half-human), infuriating American public beyond every measure.
Yamamoto himself allegedly said: "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve" and that's exactly what happened in the end, although in 1941. nobody knew how really TERRIBLE things are going to get, but that's unfortunately true for almost every large conflict in human history.