The silver-haired forgemaster had cut his hand because he doesn't want to say that he isn't loyal anymore to her (yeah you know who I'm talking about) just because the tables have turned. No, Hector cut off his finger because no matter how easy it is to unsay his loyalty he won't because he still wanted her somehow, that's why she wanted her to be saved and to have, let's assume, have a life with him, even after all that's happened. Losing his finger was just a necessary sacrifice because he was still loyal to her.
But everything had come to a shock to Lenore, the sudden attack to the castle, and the schemes of Hector, maybe that's why she thought she doesn't have a choice but to get the easy way out. She was distraught and left with no purpose (at least that's what she feels). I've read some thoughts by other people that maybe she could have helped Isaac build a better world, if only Isaac wasn't so busy with other matters and tried to convinced her otherwise.
In the end, she somewhat failed to realize is what she had left behind was permanent in Hector and for her, but like all suicides it was only a temporary solution. Still, that scene gave my heart so many pains I didn't even know I could have.
This ship sailed so hard like the TITANIC then it went down just as much. RIP FEELS.