Yeah, BW isn't beloved by everyone, and she's the only girl, and Whedon had to fight to get her included at all. Whedon won't kill her off.
The next person that everyone loves is Thor. And because of his mythology, he can survive his death, unlike Hulk/Cap/IM/Hawkeye. Plus, him dying makes the bad guy look unstoppable. I'd let Whedon do it.