I've read up to ADWD 2 and maybe this is explained, but if so, I missed it. What was the real reason that ... (spoilers for A Storm of Swords)
... Tywin made Tyrion believe Tysha was a whore, and broke up their marriage. What would this achieve? I know that he's a cold, hard character, but I can't believe that he would be so callous as to force his thirteen year old son to watch as his wife was repeatedly raped before making him have sex with her himself, just to teach Tyrion a lesson. How did it serve Tywin's ends to 1) break up Tyrion's marriage and 2) do it in such a sadistic way?
Answers from the books or the series are acceptable.