The bitterness of the Underwoods marriage was beginning to boil over and reaching a point where a move like this was pretty inevitable. Since the very first season, Frank and Claire have had some pretty big disputes and issues, but this is the furthest it has spiraled downward. And yet as Claire pens the letter to Frank, and as Frank reads the letter, neither of them show much emotion at all. Franks insistence that she must be stopped sounds like hes talking about a hated adversary rather than his wife. Something has clearly broken between them and it would take an act of divine intervention to keep them together.
