This might surprise some City fans, but a huge proportion of neutrals think Agüero is an absolute ****. They remember him for his tackles on David Luiz and elbow on Winston Reid. Sterling, Fernandinho and Tevez are just the tip of the iceberg in terms of City stars who neutrals hate. It's easy to build up a hatred for a player from a team you hate.
Also may I point out that a lot of that hatred was actually just people calling him things like 'tranny', which we would probably be better off trying to forget about.