I like this depiction of the two types of sadists here: Sadism as a kink, Doona. Finds it arousing because she knows they secretly enjoy it deep down, forcing that out of them. Though only intends for short term pain, not long lasting (most cases).
and then an actual sad***, Wookyung. He likes causing/the concept of actual pain and discomfort, likely long term destructive pain too. 
It's cool you get to see both here though, usually stories center around one type