I wonder what broke her.
First, it was Arthas and slaying her people with her own arms. That was a big trauma. It made her into this vindictive hunter. I guess at this point, she just wanted him to be dead and that's it.
Then, when Arthas was no more, she suicided and realized that there's more in this world - Forsaken. At this point, I think she was at her best time, she finally accepted who she was, her new fate and she clearly started to care more about her people, about her kingdom, maybe even a bit about Horde. I think what she wanted here is someone she could love? Who? Vereesa.
Then, Vereesa kindly declined her sister's "gift" and it traumatized Sylvanas even more. This is exactly when Legion-BfA Sylvanas was born. This is when she started hating Life in general, searching for Helya, etc.
Then, we have Delaryn triggering her to burn Teldrassil, this was point of no return.
In general, Sylvanas is just deeply traumatised person who needs a lot of therapy to let go of her sister's rejection. Vereesa is to blame, in other words