This is what happens when your favourite player joins a direct rival. No matter how dear was Cesc Fabregas to Arsenal fans, the minute it was announced that he has officially joined the Blues, things like this happen.
Although, Wenger might have turned down the opportunity to re-sign the former Arsenal captain, but the fact that he agreed to move to Stamford Bridge even before the start of the World Cup is what the Arsenal fans are pissed about. The likes of PSG, and Juventus showed interests, but he opted to move to London and play under Mourinho, the manager, who he pretty much hates (or so he has told in the past).
Here are some more pictures: