I'd say you're probably wrong on that. The Thunder are hated by Sonic fans for the most part. The Heat are generally disliked in a way that the Yankees, Duke basketball, and the Patriots/Cowboys are. Heck, the Lakers or Celtics might even be above the Thunder on the list of most hated teams in the NBA.
I'm a die hard NBA fan and have never heard of anyone hating the Thunder.
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.
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.
I'd say Heat and Thunder are tied for most hated team.