If it weren't for what DongHyuk is doing, you think KyungSoo could have any sort of normal life at all? DongHyuk is driven to despair, but it is actually the thing that needs to be done. He either kills KyungSoo or Yule kills himself, otherwise the eternal rape/kidnapping/manipulation/torture continues. This is the showdown that is needed for things to change.
So you think that what Dong Hyung is doing is right? Wtf. If Kyung Soo was in his right mind he would help Kyung Soo escape, and sacrifice himself trying to fight Yule himself. If he really loved him he wouldn't take this selfish decision of taking Kyung Soo's life, he has no right to. This not the way to solve Kyung Soo's problems, this is just madness only an unstable person would choose.
No, I think DongHyuk is off his rocker because of all he's been through (guilt, shame, obsession). But what he's doing in his nutjob state is exactly what is needed to wake up the people who are in this insanity loop (KyungSoo continually self-sacrificing and Yule continually hurting). This is the thing that's making Yule stop and face shit and maybe KyungSoo, too. Both of them are in a majorly destructive loop that they dragged others into. This wild act can break that.
No this is wrong. What if Yule actually gave even less shit for Kyung Soo and actually didn't stab himself with a knife (sorry for spoiling)? Then Kyung Soo would end up dead by Dong Hyung. The less deserving of death person would end up freaking dead.
Also that's why i said that Dong Hyuk would stay behind, and try to fight of Yule with a better plan, while he doesn't expect it so that Kyung Soo would escape freely.
That is one path, ,but we don't know if DongHyuk would have gone through with it. We only know there's a threat at this point. Considering he had TWO knives, it's possible his plan was to pressure Yule. What DH should do is just kill Yule and between him and KyungSoo, they could make a helluva case of self-defense. :)

Hey Dong Hyuk, i have a better idea. WHY DON'T BOTH YULE AND YOU COMMIT SUICIDE AND LET OUR POOR CINNAMON ROLL LIVE IT'S LIFE HAPPILY AND FULL?!