Yep, she killed Arryn, that was obvious. But the brutal killings of all the bastards was all Joffrey, she was coy about it at first, but later denied involvement, didnt want to outright say it was her dumb son.
She didn't do that, Joffrey did. Tyrion accuses her of doing it and she doesn't admit to it, he then says that she didn't know about it, so it must have been Joff. Joffrey didn't want anyone coming to the throne against him.
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The only reason I challenge you is because I've watched every episode in the past two weeks.