The United States was far from neutral before entering the war, but they were still very much an enemy. They attacked Pearl Harbor in a surprise attack (though this is debated) and were very cruel to prisoners of war and civilians in conquered lands.Japan was heavily conflicted about involving the US. The neo-cons of their day were all for total war. But the smarter and more rational Japanese leaders including Admiral Yamamoto thought that war with the US was suicide.
It was all about saving face in China. Japan could have avoided war with the US and the West by withdrawing from China in the face of the economic embargoes. But that would have been a humiliation, so they chose war with West in order to continue their war in China. Stupid, stupid, stupid.