We use 대박! for awesome thing, very nice thing, incredible thing, to emphasize adjective like totally, etc..
Usually this word used to positive thing but sometimes it's used to negative ex-pression like ㅎㅓㄹ!
But it is depends on former sentences or situation.
in the positive situation :
A: 나 로또 됐어(I won the lottery!) / B: ㅎㅓㄹ 대박! ( wow, amazing! )
in the negative situation:
A: 사람들이 다쳤어(people got hurt) / B: ㅎㅓㄹ 대박! ( really? that's bad )
Using 대박 in the positive situation is better