She fell in love with him, but her brother was being used as hostage/bargain tool.
MC guessed that which is why he didn't defend himself from her and let her kill him, even when he was under the poison he still could have killed her effortlessly because he was that strong.
This part is MTL: apparently the assassin girl lost her mind because the brother was dead all along, the guys that send her on that mission didn't expect her to succeed so they got rid off her brother at some point, now I'm not sure if she just went crazy or killed herself but I think it's the former.