For Seattle fans, it's not merely that the team left, it's how they left. Some of Clay Bennett's e-mails got leaked, and they make it clear that while he was publicly promising to try to keep the team in Seattle (which was a condition of the original sale) he was privately saying he'd move the team to OKC ASAP. As long as Clay Bennett is involved with the Thunder, the majority of Sonics fans are going to be against them.
Also remember that of current Thunder players, only Nick Collison was on the team before Clay Bennett arrived. Kevin Durant played one year in Seattle but it was already common knowledge that the team was going to leave, so fans didn't really take to him much. All of the other rotation players were added after the move. So there aren't any beloved players left on the team to mitigate the anger coming out of Seattle.
It's similar to LeBron being thought of as a villain. It's not that he left Cleveland, it's how he left Cleveland. To a lot of fans in both cities, it felt like their guy/team lied about wanting to stay, left under dubious circumstances, and disrespected the fans and the city on the way out.
Also remember that of current Thunder players, only Nick Collison was on the team before Clay Bennett arrived. Kevin Durant played one year in Seattle but it was already common knowledge that the team was going to leave, so fans didn't really take to him much. All of the other rotation players were added after the move. So there aren't any beloved players left on the team to mitigate the anger coming out of Seattle.
It's similar to LeBron being thought of as a villain. It's not that he left Cleveland, it's how he left Cleveland. To a lot of fans in both cities, it felt like their guy/team lied about wanting to stay, left under dubious circumstances, and disrespected the fans and the city on the way out.