How did the Malaaikah know that the children of Aadam (Alayhis Salaam) will cause mischief on the earth?

Q: My question is that as per reference of Surah Bakra when Allah Ta'ala created Hazrat Adam (Alayhis Salaam) then the Angels said "this man will create fights on earth"(mafhoom). So how did they know that because before Hazrat Adam (Alayhis Salaam) they never saw any human being?

Speaking lies

Q: Married couples go and live abroad. The husband legally divorces his wives and makes her live like a single wife, divorcee or as a widow in order to get monetary assistance from the government. Is this permissible?