He was hurt by it. I don't think he realised how badly they would take him going to United as several Arsenal players had left for City without much backlash, I think he expected something similar.
The fact that Arsenal fans crucified Robin who left for United because he wanted to win a PL title, but didn't care about Adebayor, Toure, Clichy and Nasri going to City for the money at a time when Arsenal were better than City is weird. Never understood it. I guess it's just a United thing. He deserved better from their ungrateful fanbase.