Mad Ferdinand

I find it pretty interesting that all of a sudden Ferdinand is sick and in need of a doctor's attention. He seems to be a little off (being afraid of his own shadow), and I think its caused by the overwhelming feelings of guilt he is suffering for his sister's murder. His madness is our final proof that he does have feelings of remorse for what happened to the Duchess. But what I don't quite understand is his reason for killing his brother. He just stormed into the room, saw Bosola stabbing the Cardinal, and he himself attacked the Cardinal. I expected him to attack Bosola for stabbing his brother. I don't understand the reason for his actions. Perhaps it is because the Cardinal brought Bosola into the picture in the very beginning, and Ferdinand is trying to put more blame onto anyone else but himself because he can not stand the guilt he is suppressing.