How come the right answer is "aga dunia"? Someone help.
It's an idiom. It literally means "say goodbye to the world" ...
... it should also accept "kufa" and "kufeni" thought.
True.'Kufeni' when you want to mean the death of more than one.
That too is correct.However,the most commonly used term is 'kufa'.
aga dunia is extremely common as well
How casual is this? Could you say it to a friend in mock anger, or is it more serious, like something you say before actually killing someone?