mmm yes but i see it more as his innate tendency to hurt people (in every sense) more than anything, not necessarily sadism per se. in the end he's able to discern between his own desires and what is actually good for garam. obviously there's a big line between sadism and rape but even when garam gives him the green light to do stuff, we can still see how heejae hesitates and outright leaves when he realizes he's hurting/could hurt garam

i doubt heejae actually wants to hurt garam... heejaes main big flaw as a person is him not being able to correctly communicate (both to other ppl and himself) his sincere intentions. he's a broken man with very, very deep-rooted problems but in the end both him and garam share that guttering, heart shredding need of wanting to feel loved for who they really are.... him "wanting" to hurt garam is his way of thinking "i want you to demonstrate that you love me", which is exactly what garam asks of him every 9 seconds
it may be hard to understand why garam is attracted to him but i feel like he's not only obvs #entranced by that #oldman#cock he's also fascinated/drunk off realizing the prof shares the same insecurities and vulnerabilities he has... despite their differences they mirror each other in a lot of ways