Why do people hate the Thunder?
I can see the argument that some Seattle fans would resent that the team left, but I think their anger would be directed toward the owners involved or David Stern - not the team itself.
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.
To me, the Thunder have no right to exist and no right to win anything. Any success for them is nothing but an insult to Sonics fans.