I just watched "Marriage, Not Dating", and I'm currently watching "High Society".  It just so happens that both have such messed up characters with no morals that it's hard to watch some times.  I'm really curious about more of these types of shows with love "hexagons" (instead of triangles), corruption and infidelity.    

Our Gap Soon was so frustrating some of the characters were so out there. Also Daily dramas have ridiculous plots and often a cheating spouse for either the lead or a close relative of the main characters as a side story. Not quite a hexagon though.
Maybe try Go Ho's Starry night it was reverse harem with an exboyfriend as 2nd male lead, main lead is a boss with an unrequited love for female lead and there are other guys confessing their interest for her too. 

Any drama that has assault in it, ESPECIALLY if the assault is overlooked and diluted into a forgotten mistake, and not something that speaks to the utter lack of humanity in a person. Besides that, the usual makjang elements of dailies and certain longer dramas are always pretty messed up (like horrible parents trying to control every aspect of their childrens' lives thus making them miserable...can extend to anything from marriage to murder). Sometimes you see a window into how deeply patriarchal society still is and how women, despite how they are as individuals, are genuinely treated as nothing but child-bearers and maids is horridly frustrating, and just as much unfair. I think one specific example that stands out among others in my early drama watching (re: just a lot of messed up elements) was Que Sera Sera, like, was all that happened really necessary lmao? Also anything fauxcesty that borders a little bit too much on the -cest part.

Cruel Romance. Couldn't finish it because it just felt so wrong. ML was so domineering over the FL and she allowed him to be??? That part that made me drop it- the ML was horny af but the FL wasn't ready for sex and he just pushed for it. Huang Xiaoming's good looks can't even save his trashy character. Smh.

Kuzu no Honkai was messed up, all the characters were mentally unstable people

Secret/Secret Love starring Ji Sung -many of the characters in that drama were messed up.

Atami no Sousakan is pretty messed up, but in a good way. 

I agree with Kuzu no Honkai, like seriously I put it on hold because I cannot take it. Even when Dori is there. haha

Black for me. Lots of loopholes actually.