Here's what actually happens:
* Bane's death is a result of the collapsing Gotham Stock Exchange building. The building collapses due to a pre-programmed detonation set by Bane himself as part of his plan to destroy Gotham.
* Selina is present during the collapse and is seen leaping off the building in a last-minute escape, leaving Bane to his fate.
* She does not actively kill him. She merely survives the same situation that ultimately leads to his demise.
It's possible that some viewers might have interpreted the scene as suggesting Selina was somehow responsible, but the film makes it clear that Bane's death is an unintended consequence of his own plan.