If you want to commit easy kills simply use a knife. When using the silent kill knife not only do you kill the guard but you don't attract other guards in the area.
To unlock bonus gallery pictures at the end of the game make sure to complete all of the missions secondary objectives.
To unlock cheat mode select Options at the main menu, then select game. Press Right, Right, Left, Right, Left, Left, Right at the Game Options menu. Doing this correctly will allow you access to the debug game mode.