Men Behind the Sun isn't really horror either,it's more "historical-exploitation" (which may or may not have been the intent of the filmmaker) like other HK/Chinese films concerning the Japanese horrors inflicted on the Chinese (think Comfort Women and 1941 Hong Kong on Fire for example).
I'd recommend having a look at Lost Souls (1980) to start things off,it's an (again) very explicit (and exploitative?) social drama about refugees from mainland China who are taken prisoner by ruthless slave traders.
This was made when he was still working for Shaw Brothers btw.