Overwatch players have requested a deathmatch game-mode since the game very first launched, but the team at Blizzard and Jeff Kaplan himself have always strongly believed that Overwatch just doesn't mix well with that kind of gameplay.
Team deathmatch or free-for-all deathmatch is basically the oldest game type in FPS history, and although it is highly enjoyed and loved, Overwatch focuses more on teamwork, compatibility, and objective based gameplay. We'd never thought we would get any game types even resembling Deathmatch in Overwatch, until the introduction of Arcade, which allows Blizzard to release fun game types, without hurting quick or competitive play.
Finally, via a developer update, Jeff Kaplan has announced the introduction of deathmatch into the Overwatch arcade. Pick whoever you want in this free-for-all deathmatch. Finally, Hanzo mains can shine without getting blacklisted by their teammates. Sadly for Genji mains, they won't be receiving any healing in this game type.