While it could be the obvious answer of a fascination with Nazi culture, I doubt it. I don't know about Korea, but in Japan, they have what appears to be a swastika on most shrines. It's actually a symbol that used to mean something good, but they generally stopped using it during/after WWII. Since Korean and Japanese have a whole different alphabet and Korean doesn't inclue that symbol, I'm not sure how relevant that is. Also, the swastika used to be the symbol for an organization to a group similar to the American Red Cross. But really, I'd just ask your roommate if it really bothers you that much.