Successfully complete the game on any difficulty setting. Start a new game, enter the "Options" menu, then select "Cheats". The following cheat options may now be activated:
Extreme Ragdoll
Hippy Blood: Changes spilled blood into colorful flowers.
Low Gravity Ragdoll: Enemies will fly away when killed. Put them in orbit by destroying their oxygen tank.
Level select
Successfully complete a level to unlock it at the "Cheats" menu.
All weapons and upgrades
Successfully complete the game. Instead of starting a new game, use the "Level select" cheat and start on the first level to keep all your weapons and upgrades from the previous game.
Massacre mode
Successfully complete the game on any difficulty setting to unlock the Massacre difficulty setting.