Maybe I forgot smth but why would him being tied uk trigger him?
In my opinion, Sekyung feels super guilty for how he left things with Yiheon. I mean, Yiheon did commit suicide after all and I think Sekyung blames himself for it. I'm willing to bet it's to the point where Sekyung is having nightmares about it and can't even sleep. Otherwise, he wouldn't be stalking and harassing Deukpal to such and extent. To simply put it, he is desperately trying to make amends with Yiheon. If he can just see that Yiheon is happy and well, maybe that will alleviate some of his guilt. So the last thing he'd want to see is Yiheon getting hurt.
Just imagine that you epically wronged somebody. Like, they didn't really do anything wrong to you and you screw them big time. And while you walk about, you see them getting bullied and suffer even more. If you see that, you'll feel even more like a d!ck for screwing that person when they're already having it hella bad.
Like I've said, I think that Deukpal only said those things to annoyed Sekyung. He didn't really mean anything serious by it. But it was his words that brought that image to Sekyung's mind. The thought of seeing Yiheon tied up and crying will bring about a whole new kind of nightmares. Deukpal may think it's nothing but to Sekyung, it's f@cked up. I think that deep down, Sekyung knows that Deukpal is just messing around. But those words hurt him deeply and his immediate reaction is to do the same thing to Deukpal. Casually making accusation and accusing Deukpal of kidnapping and murder in attempt to hurt Deukpal back.
That comment about Yiheon being tied up somewhere triggered Sekyung hard. I think Deukpal is probably annoyed at Sekyung's persistence and is just making those comments to annoy Sekyung back. But to Sekyung, the thought of Yiheon being tied up somwhere must have been upsetting. Now, he's lashing back at Deukpal, calling him a kidnapper and a killer to hurt Deukpal's feelings. Let's see how Deukpal will react. Will he just brush it off like he always do or is he gonna punch Sekyung in the face?