She knew that if she got near the flames that they would encircle her and kill her, quickly but very painfully. But she cast the spell against Grindelwald anyway, knowing that she was about to be killed, but she hoped that she'd be able to protect Theseus and Newt from the flames and the monster that had created them.
She felt guilty for what she was about to do, she could tell that Newt was friends with the blonde woman who had joined up with Grindelwald, but she couldn't, she wouldn't, live knowing that she could've protected them in some way, even if it meant sacrificing herself.
That's when the idea struck her. Fake her death, for now anyway. Make Grindelwald think that he had gotten rid of her, he saw her as a threat. Her and the blonde, that's why he had gone to those lengths to recruit them both.
The blue flames encircled her just as she had predicted. She turned to Theseus and Newt. "I love you." Even she wasn't sure which of them she spoke the words to, deep down she loved them both. And she allowed the flames to burn away at her skin for some time, the pain was unbearable, but she took it anyway knowing that this would buy them all the time she could. After a few moments, she dropped to the ground and disapparated. Headed for the only place she could think of, Leta Lestrange disappeared, not sure herself what she would do next, but knowing that Grindelwald would never see her coming.