The following are the ten types of kills you can get:
Sword Kill Trap Kill Poison Kill Water Kill Explosive Kill Spike Kill Tiger-Trap Kill Fall Kill Surprise Kill Joke Kill.
In order to get Surprise Kills and Joke Kills, you need to unlock the scrolls that allow you to make Joke Books and Surprise Boxes. Once you get kills with all the different types, the background of that enemy's kill gallery will turn gold.
To easily get scrolls, fill your inventory with bamboo traps. Then, line them up one after another in the path of an enemy so that he will hit the first one and bounce to the next one until he has hit all eight traps. If done correctly, each time the enemy bounces off a trap he will drop one of the following: money, ninja gear ingredients, or sometimes a scroll. Also, you can sell the eight trap kills you just got at the shop for money.