After finishing any level, you get rating. It can vary from 1 star to 5 stars. To get maximum rating, you'll need to:

• Complete level on Hardcore difficulty (1 star)
  
• Keep within certain time (1 star)
  
• Perform execution as much as possible, notice that they must be violent, this means you should hold attack button until red crosshair appears (3 stars)

There are 4 bonus levels in total: "Hard as Nails", "Brawl Game". "Monkey Sees, Monkey Dies", "Time 2 Die".

To unlock them, you'll need to score 3 or more stars on next levels:

Completing each level with 3 or more stars rewards you with one artwork bonus. Each two consecutive artworks depict half of a cheat code that can be used only if these levels have been completed with 5 stars rating. Some of the artworks are removed content from the game.