How did the Japanese treat the people they conquered?

Absolutely horribly. With extreme cruelty, barbarity and savagery. The men were sometimes forced to work for the Japanese as slave laborers. Women were raped and then killed, or forced to be "comfort women" (prostitutes) for the Japanese Army, in Army-operated brothels. At least they were not murdered immediately. Everybody else in Asia despises the Japanese to this day, not just for what they did, but for the fact that they won't admit it, and see nothing wrong in what they did. See the "Rape of Nanking", where Japanese troops were allowed free rein for six weeks after capturing that Chinese city. 300,000 Chinese civilians died in those next six weeks in Nanking.