From the Super Mario Wiki
Killer Ball is the name of the Special Ball that Bowser uses in Mario Superstar Baseball. While pitching, Bowser is able to throw a Bullet Bill at the batter. The Bullet Bill moves slowly towards the player, flips around, and then flies past the batter at high speed.
If Bowser is batting, he is able to turn the ball into a Bullet Bill once contact is made. If caught, the Bullet Bill pushes the catcher far back. The only difference between Bowser's Killer Ball and Jr.'s, is that the Killer Jr. Ball wears a bandanna, just like Bowser Jr. himself.
Mario Super Sluggers
The Killer Ball returns as Bowser's Star Pitch move in Mario Super Sluggers. Just like in the predecessor, Bowser will throw a Bullet Bill that the batter must hit, but the Bullet Bill will do a loop before reaching the batter, which can mess up his or her timing.