Ace was a robot member of the third Royal Flush Gang.
The first heist his Royal Flush Gang did was attack the passengers and steal their possessions. He was later destroyed by Batman.
The destruction of Ace was one of the few times a character was "killed" in DCAU, albeit not an actual murder, rather the destruction of a droid. This was also seen in Batman: the Animated Series "The Last Laugh" when Batman destroys the droid Captain Clown, causing Joker to shout Batman "killed" it.