Baseball bats are used in baseball games. They are also sometimes used as weapons.
If you dream that you are playing baseball and holding a baseball bat, you are experiencing competition in real life. The way you perform on the baseball field reflects the way you believe you will handle your competition.
If you dream that you are holding a baseball bat to use as a weapon, you are repressing anger about something in real life. Your dream is telling you that you need to deal with the problem that is causing you to become angry.
If you attack someone else with a baseball bat, the person that you attack in your dream represents something that is causing you to become angry or frustrated.