Bellatrix laughed at the pain on Harry's face and she smiled at him with her tongue hanging out as she could feel the excitement now radiating from Hermione's own magic. There was nothing left except to kill Harry as the boys had done the work of destroying the other Horcruxes for them unknowingly that Voldemort was already dead. This just meant there could be no backlash from him in the future and they could rule now as they see fit.