Arabella was grateful that her husband wanted to be honest with her despite the risks of getting turned down. He was being sincere and didn't want her to agree without knowing what she was agreeing to.
Thus, she wanted to respond in turn and be honest with him too. After all, she truly wasn't the kind of pure soul he thought either.
"Ferdinand, I know I am weak and you are strong. But there are other ways to kill. I can plot your death and let you die without having to do it myself. And you are wrong if you think I am a pure innocent girl. I have killed people. I have a ton of lives on my hands. I didn't stab them myself nor slit their throats but it's my fault they died. I plotted their death so it's the same as me killing them. Are you sure you still want a wife like me?"
'Why am I saying all these? What if he changed his mind?'
