I seem to recall that Jon Connington lost the Battle of the Bells because he wasn't ruthless enough.
That's what make Jon Connington likable...
In one of Connington flashbacks he was told if Lord Tywin was fighting Robert at the Battle of the Bells, the resistance would have ended there... Tywin would have presented Ned and Jon Arryn Robert's bones because he would have burned down the entire town...
And Tyrion also made a telling statement about Lord Tywin... He said Danny knew nothing about sieges because if Lord Tywin was besieged like Danny were, he would have poisoned every water well within miles of the city and let the sieging army die of thirst and disease... Even though it would have cause the death of many innocent people as well.... But he would have won...
That's why Danny need Tyrion councel if she plan on winning against the mercenary armies... She is surrounded by honorable men but not military minded lords...
Even Jorah said Drogo would have had a very hard time fighting against men like Tywin or Ned because they would sit behind their castles walls and wait out the invaders unlike Robert who would meet them in open fields...
Lord Tywin had to die because Danny would have lost against him just like Robb did... Her Dragons isn't big enough to really turn the tide of battle in a major conflict.
Lord Tywin was smart, ruthless, experienced, and had a lot of money...
Everyone talk about how peaceful and prosperous the seven kingdom were when the mad king was ruler... Everyone said the only reason the seven kingdom were prosperous is because Lord Tywin kept everything together. He ran the seven kingdom... And the minute he left, the kingdom fell apart and into war...