Prior to trying this, you must have at least beat the game on POSTAL 2 game mode (Monday-Friday), unlocking Enhanced Game.

From the main menu, pick New Game, then input the following code via the Directional Pad (on either your keyboard or gamepad)


UP UP DOWN DOWN LEFT RIGHT LEFT RIGHT (It's the Konami code from the Nintendo games!)


And voila! The following options explained:
NPC Difficulty: Higher values make the NPC meaner and tougher. 1 is the equivalent of Liebermode, 10 is the equivalent of Manic and above.
Violence Rating: Determines the type of weapon they receive. At Ludicrous, they receive the best weaponry possible including the WMD but no Nuke Launcher.
They Hate Me: All armed NPCs hate you on sight.
Expert Mode: Disables storing health pickups and allows one save per level (cannot be used with Enhanced Game)

See what kind of ♥♥♥♥♥♥ up difficulty you might come up with and enjoy!

LeonardoW for finding out about it.
RWS Rick F for extra info.