Now, the Freys are placated, the Karstarks are revenged, and Roose Bolton is the big dog in the North. Not to mention the War of the Five Kings is basically over now.
It's even sweeter for Tywin. He set the Freys up perfectly, because the way they did things they broke the Guest Rite of Westeros and have disgraced their name for the rest of history. They took out HIS enemy, and in the process ruined themselves so everyone will blame them first and foremost and ignore Tywin as the main man behind it all. Tywin gets away with his name intact, the Freys are utterly screwed and will the the target of all revenge schemes from now on.