Matches Malone

The Waynes' killer is the unnamed murderer of the parents of Bruce Wayne, Thomas and Martha and is a minor yet important antagonist in the Batman prequel TV Series Gotham. He is played by Danny Schoch.

Gotham
When Bruce Wayne is leaving the movies with his parents Thomas and Martha, they cut into an alleyway. The masked man approaches them and points the gun. Although they comply with his demands, he shoots dead both Thomas and Martha. He hesitates shooting Bruce, but instead puts away his gun and flees off into the night.

The killer is yet to be caught, or most importantly why Thomas and Martha were killed. A man named Mario Pepper was set up for the murder by the mob in order to give hope to the city of Gotham.

Trivia

 * In the comics (and a majority of the Batman extended universe) Joe Chill is responsible for the murder of Thomas and Martha Wayne, which is sometimes depicted as a mugging gone bad, and in others on purpose. It is unknown if this particular killer in Gotham is called Joe Chill.