Honestly, that's the only question I have. Why??? Sunjin is literally using him and he SAID that. And DS relationships in manga are usually talked out by the two. They explain their kinks, ask for permission, and they have a safe word (I know they had one but they never used it, even when he was uncomfortable). This would have been 1 MILLION times better if he had ended up with the reporter lady or even just committed suicide. Very disappointing, to say the least.
I WAS SCREAMING BRO I WAS LIKE "NO NO NO NO NO TELL HIM TO COME BACK TELL HIM TO COME BACK NONONONONONO PLEASEEE" then when he brought him to the cliff I was like "YEEEEEEEEEESSS- WAIT NOOOO-