as much as Yu-Hon killing Kashi is obvious I think we still can hope for a plot twist.
Like what if it is atually Il who killed her ? we never sow them as a lovely couple, so far he protected her for her power. what if she sow in the future that Yu-Hon will be an enemy therefor Il killed her make a great excuse for him to get ride of his brother.
than again Yona's father didn't seem that bad of a guy to kill his wife, but this chapter imply that Yu-Hon was killed becase he killed Kashi, which means that Il either killed his brother to avenge his wife, or killed hid wife to kill his brother