Deeney, a boyhood Birmingham fan, stuck the knife into the team he loves to score against more than any other asWatford ran out 3-2 winners.
Relegated Villa were leading with just five seconds of normal time remaining thanks to goals from Ciaran Clark and Jordan Ayew, either side of Almen Abdi’s free-kick.
But the beleaguered midlanders, who had Aly Cissokho sent off with 15 minutes left, could not hold out as Deeney’s double piled on the agony.
The match initially settled into a miserably familiar routine for Villa, camped in their own half while being jeered by their own fans.
Watford striker Odion Ighalo, playing despite the death of his father last weekend, could have twice opened the scoring but for a block by Joleon Lescott and a sharp near-post save from Mark Bunn.
Hornets midfielder Ben Watson was then denied a goal-of-the-season contender when he flicked the ball up some 30 yards out and launched a dipping volley which crashed against the crossbar.
However, after 25 minutes Villa made it into the Watford half, and even managed a shot at goal.
Although Ayew’s effort was way off target the away fans launched into a mock celebration as if they had scored.