How do I say 'You're so bad.' in Korean
(informally) in a playful way towards close friends or family.

Question

Answer

You could say 넌 참 [정말/너무] 나빠 (You're so bad) with a playful tone.

Some other expressions:
- 너(는) 정말 못됐다:  You're so bad [no good]. 못되다:  bad. ill bred.   
- 너 참 잘 한다:  You are doing great (sarcasm) 
- 너 정말 대단하다:  You're so great. (sarcasm)