The 2nd worst thing the First Order did was destroy the 5 Republic Planets.

When I saw this, I almost cried. It was... just really really sad.

The new Republic  I think is a new government or something that became a thing after the Empire was destroyed. They were secretly helping the  Resistance *the people fighting against the First Order*. The First Order hated this.  So, they used their super weapon, and literally destroyed all the Republic planets. Everyone on the planets, died. Probably billions.