What Newt didn't know (and thus Queenie didn't know) was that Leta Lestrange had known when she'd dug too deep into the dark side. Leta had known that there was no hope for her. Leta had known that those who loyally stayed close to her would be doomed like her.

Leta Lestrange broke her loyal badger's heart in the most publicly humiliating way possible, because otherwise he would have followed her to hell itself. And Leta Lestrange is not sorry for that.

(OP has absolutely NO IDEA what Leta got herself into, but thinks it might have been Eldritch Abominations.)