According to the July, 25, 2014 Patch Notes, this was added:
Kill Streaks
A kill streak meter has been added next to the in-game Hero portrait.
Notifications are displayed at the top of the screen when a player achieves kill streaks of 5, 10, 15, and 20.
Players’ nameplates now light up or catch fire to show off when they’ve achieved lengthy kill streaks.
Kill streaks are reset upon death.
There is no mentioned of any sort of reward, except that they light up.