Harry suffers grievous wounds that make him almost die. Fleur thinks he does and something in her breaks. She shifts into a monstrous and supremely beautiful form that is beggeted to her by her blood, in her fractured mindset she murders a death eater inrevenge. It's easy. She listens to the voice of Harry on her mind and realises he is not dead, but in a sort of comma, but despite that she can hear his voice guiding her.
She swears an oath of revenge and cleansing. She is going to kill Voldemort and all his followers if it's the last thing she does, and with the light of the best of mankind as her north star she is gonna succeed