GIVE ANSWERS - English

Vote now!उत्तरLanguage Question
When do I have to use "shall"?

For what I know shall is another way of future, isn't it? but while watching shows I have listen the phrase "shall we?" I understand the context of the phrase, and in one of my favorite shows the sentence "I shall destroy your happiness if it is the last thing I do" in my head that sounds like "I should destroy your happiness if it is the last thing I do". And, it is shall only uses in a formal way?