No, no one really hates japan because of hirohito. However, some hate japan because of the Rape of Nanjing. Koreans aren't fans due to the occupation of Korea, which basically turned half a mil Korean women into sex slaves, destroyed historic sites, nearly wrecked their cultural heritage, and resulted in nearly a million dead over the 35 years they held power there. Sure, they're all anime cat girl maniacs now, but that's not what they've always been, and there are a lot of people with loooong memories.

They don't hate Italians because of musollini, but some Ethiopians still may hold a grudge over the Yekatit 12 massacre. Or because of what they did in Libya right before wwii.

People don't hate other groups because of the leaders. They usually hate them for the atrocities they've done, just like the atrocity done to a navy ship called the uss Liberty.