Kill Anyone without Losing a Life:
When you have beaten the Game you can replay Memory Blocks in Story mode and can kill anyone you like without being penalised.

Unlock Secret Ending:
When you have beaten the game and the credits have completed you will be able to view a secret ending.

Loud Assassination:
Hold LB + RB and press X to perform this type of assassination which can be used when approaching a guard, the only problem is that because it's noisy it will alert other guards.

Silent Assassination:
Hold LB and press X to perform this type of assassination which is most effective when used from behind a guard as nobody will hear a thing.