As such, Shae's appearance at Tyrion's trial is just another brick in the edifice of Tyrion's guilt. That it will hurt Tyrion is (largely) beside the point. Tywin's loathing for Tyrion consistently causes him to disregard Tyrion's feelings, and it never occurs to him that Tyrion would behave so irrationally as to turn down his chance at survival. More @Wikipedia
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