There are some war/dystopian movies when, as some sort of violent regimen grows, former friends or acquaintances - who usually have opposing viewpoints to the protagonist, to some degree - end up becoming part of the regiment, usually as officials or authority figures of some sort.
Some examples:
- "The Pianist" - a friend of Szpilman becomes a Ghetto Police Officer;
- "Dr. Zhivago" - Pasha Antipov becomes Strelnikov;
The trope poses a symbolic question, IMO, as to the extent to which political divergences between you and some person you know are potentially dangerous if the power balance is disturbed.