They're both fucked up and they both fucked up.

I was slightly more-so on Dohyun's side up until this point because we hadn't been getting any backstory on their history together before, but by this now (ch.30) it's clear to me personally that Sungho's "revenge" is entirely unjustified. Of course this perspective is subject to change, but I need more information first.
From what's been revealed, it's Sungho's own fault that he ended up going through those things because whenever Dohyun would back off in response to Sungho's refusal or discomfort, he'd retract and submit to him in order to keep him from leaving. Yeah, Dohyun's a selfish ass hole for only wanting Sungho for sex, but ffs Sungho knew that and still went out of his way to get used because obsession > discomfort.
He basically carved himself with the knife called Dohyun, and now he's blaming the knife for the scars instead of the one who used it.

tl;dr I'm tentatively on Dohyun's side until more is revealed about their history since Sungho had the choice and knew exactly what he was signing up for.