@uMoonlight The thing is he tried to convince her to not go along with the execution but feeling betrayed by him and with all her family dead, she basically demanded being executed. She even went to the execution in full noble jerkass mode (something that she was never before) to ensure that all the evils of her family died with her. It's not like he chose to kill her out of revenge, malice or whatnot, she chose to die and the people demanded her death by association. So she was executed.
And regarding her... I think that what she has is a depressing mix of both Bystander syndrome and Bloodline crimes. She feels that just because her family was doing such evil behind her back, she had the duty of both investigate what was going on and to stop them. Thing is, she never questioned where all the luxury she lived came from and her family never allowed her to mingle with the populace. She only discovered it when everything blew up and was imprisoned, well after she could do anything about it. This perceived failure in stopping the evils of her family and feeling a culprit herself just because being a part of that family is what drives her depression.
All of this is in the first chapter. It's... depressing to say the least.