Why on earth are they doing like that?

Why on earth are they doing it like that?

Please correct my English:))

오늘 점심은 간단하게 김밥으로 때울까?
Shall we simply make do with 김밥 for lunch today?
이 게임은 혼자 시간 때우기에 좋아.
This game is good for killing time when you are by yourself.

하루 종일 트위터 하는 사람들이 있어.
Some people use Twitter all day long.
도대체 왜 저러지?
Why on earth are they doing it like that?

뻘짓 그만해. 
쓸데없는 짓 그만해.
Stop doing useless things.
세상에서 제일 쓸데없는 짓이야.
It’s the most useless thing in the world.

그럴 시간에 한국어 공부했으면 벌써 다 뗐겠다.
If you had spent the time for studying Korean, you’d have already mastered it.
그럴 시간에 한국어 공부했으면 벌써 끝났겠다.
If you had spent the time for studying Korean, you’d have already finished it.