How do you figure this would have stopped his child or grandchild or Japan's military or other institutions from perpetuating fascism?

Korea was already under Japanese colonial occupation. Its formal annexation in August 1910 would have to have been consented to by the Taishou emperor instead, but I doubt that would have stopped it. And the Taishou emperor would have died around the same time leaving the Shouwa Emperor (Hirohito)'s reign the same. It was under Shouwa that Japan invaded Manchuria and later the rest of east Asia.

Don't get me wrong, assassinating one emperor is partway to assassinating the entire imperial male line which would significantly change Japanese attitudes because of the religious implications, so it was worth trying. But this one thing would not have prevented it AFAIK.