I am reading the second book. I am on chapter 46.

I have a question of Ramsay Snow. I am a bit confused as to what happened. He supposedly poisoned Roose's legitimate heir while Roose was fighting in the War and has a death warrant of some sorts issued by Ser Rodrick Cassell at this point in the second book? I may be confused about this whole story. Who was Roose's heir supposed to be and why would Ramsay want her dead? I remember Bran being told of this, I am just confused as to the whole beginning of this story I believe.  Anyone wish to summarize and clarify for me?