Watford were leading 1-0 when Mariappa was sent off – Hornets captain Troy Deeney reacting quickest to slot home after Tom Heaton had palmed away Abdoulaye Doucoure’s shot following a mistake by Ezri Konsa.
Deeney doubled the lead from the penalty spot after he was fouled by Douglas Luiz before the impressive Ismaila Sarr added the third from Etienne Capoue’s cross.
Villa were upset the penalty for Watford’s second goal was awarded as defender Matt Targett was down injured, while the visitors were also aggrieved about the third goal as they felt Jack Grealish was fouled by Capoue in the build-up.
However, Sarr’s goal was allowed to stand after a video assistant referee (VAR) check.