Liverpool captain Steven Gerrard put his side head with a stunning volley in the 73rd minute, but Aguero -- whose injury-time goal against Queens Park Rangers in May ended City's 44-year wait for a league title -- embarrassed Reina soon after to leave the visitors in seventh place.
"Aguero's finish for the second was unbelievable but from our perspective we are bitterly disappointed he was in that situation," said Liverpool manager Brendan Rodgers.
"It was a mistake, everyone is clear on that, but I can't fault the players."
Tottenham closed to within a point of third-placed Chelsea with a 1-0 victory at 10-man West Bromwich Albion on Sunday thanks to Gareth Bale's 11th league goal this season.
However, it was marred by an ankle injury to striker Jermain Defoe, who will miss England's prestige midweek friendly against Brazil.
Ninth-placed West Brom had Goran Popov sent off for spitting just after halftime.
"The game changed with the sending-off. I'm absolutely disgusted with Goran's behavior," said his manager Steve Clarke.