That's just it though it you are not a United fan (and I say this not having a go or starting an argument), in a totally unique and likely to never happen again situation having the Sunderland fans do Man Citys celebration to rub our noses in it was the icing on the cake on what was a nightmare few minutes. Still makes my blood boil even thinking of it. Really was one of those in the moment kind of things.
Similar to Figo signing for Madrid incensed Barcelona fans to everyone else it was great drama/viewing etc. (I cant think of any other Barcelona controversies off the top of my head). Id like to think I wouldn't have thrown a pigs head on the pitch if I was a Barcelona fan at the time but cant say that in the moment I definitely wouldn't have.
Something like those situations would obviously affect fans of that club directly while if you weren't there at the time its hard to say how you would react. (not you personally just in general).
Anyway like I said not having a go and if you can say it wouldn't have bothered you in the same scenario if it was you substitute City for Madrid/Espanol and Sunderland for Villareal/Deportivo or whoever then fair play.
I'd be annoyed for a day or two but after that, i'd move on, get over it and get on with life.