Regicide

The King-Killer is an aggresive piece. It is able to move twice if the second move would capture a King. Therefore, it is good late game and against opponents whose armies include a lot of Kings.

Interactions with Kings
King-Killers can use their ability to capture both bought Kings (when the King is bought with points at the beginning of the game) and pieces that were kinged. This counts for pieces with other abilities that made it to the backrow and were kinged. That means that a King-Killer can use it's ability against kinged Leap Frogs, kinged normal pieces, kinged King-Killers, etc.

Basic Plays
Kinged King-Killers become even stronger because they have good range and mobility. Generally, if a King-Killer has been able to reach the other side of the board, some of the opponents pieces have also been able to do so, and the kinged King-Killer is able to easily "hunt" them down.