Reverse-Flash

Nora's murderer is the unidentified man dressed in yellow who was in Central City during the childhood of Barry Allen, and who murdered Nora Allen by stabbing her in the chest with a knife.

He later appears and threatens the life of Iris West by stabbing a similar knife into her photograph, to scare her father away from investigating the Nora Allen murder.

This happens shortly after Joe shared his investigation with Harrison Wells.

Trivia

 * the episode The Man in the Yellow Suit is an accurate description of this character
 * The man in yellow resembles the suit worn by various characters called the "Reverse Flash" (first "Professor Zoom", then "Zoom"), villains who share the Flashes' super-speed.