being very honest I've met a lot of people like this. it's a persons own selfishness which makes them wish things like this. taku wished that upon haesoo because he probably wanted haesoo to be just as hurt as he was.
remember through takus p.o.v the guy he loved oh so much, whom he also was sexual partners with left him to be with the guy he always loved. and during the time they spent together haesoo's eyes were never on taku but always on joowon. it's like knowing the person you love so much would abandon the world for someone else rather than you.
it hurts a lot knowing that no matter how hard you tried you were never able to keep someone close by you.
this!! remember when joowon and haesoo met up again? joowon told haesoo "no hard feelings. congratulations on your exhibition" and smiled. and later he said in his head that he didn't really mean his words and that he was hurt. but he still smiled for haesoo and told him not it was okay. despite all his hurt feelings. to me that shows the development and maturity that joowon went through,, that taku failed to show.
throughout reading this entire story, I was solely focused on taku. For me I wanted to see taku happy. After reading some of the comments which pointed out that taku was manipulative I can agree that he was. He did insist for haesoo to begin a relationship with him and at the same time haesoo did agree to it.
when it comes to feelings some persons try their best to hold on to whatever they can. I belive that taku knew that haesoo may never choose him (remember he referenced to the candy in the jar. in the end, taku never got the candy) But all taku could do was hope that the little time he spent with haesoo could change his mind.
however at the end of the day haesoo only had feelings for joowon.
*a bit of a spoiler into chapter 99*
I saw this from a previous comment and it said that taku told haesoo that he hopes that he suffers from breaking up with him. Please do remember that taku is suffering from a broken heart and it is expected that some people wish that people regret leaving them.
I think that's what taku was trying to say.