Sorry if this has been beat to death as a topic before, but I'm fairly new to these boards and haven't seen much on it
I have absolutely no problem with Tyrion killing Shae. Granted, it wasn't the actions of a sane individual, but since when is that what Tyrion is?
We are talking about a guy who has just been abandoned by everyone that he knows. Everyone sees him as this monster who selfishly killed his nephew to take the throne for himself. Top that off by throwing in that the woman he loved lied during his trial and said all these things about him raping her and wanting the throne for himself.
I'm aware that Shae's interest in him was feigned, and he probably knew that too, but that in no way gives her the right to do or say what she did. She knew how much he liked it when she would call him "her giant of lannister", and she just throws that in his face.
On top of all of that, he is (rightly, in my opinion) about to go kill his father for giving him the "lesson" that he got when he was 13. And who does he find in his bed, just the girl he has loved since the middle of aGoT.
I'll allow that emotion does get the best of everyone and causes us to make rash decisions. And like I said, his actions weren't indicative of him being sane, but I, for one, was glad that he killed her. As I read, I couldn't help but side with Tyrion on this one. I know she was a whore and was only doing her job, but her betrayal didn't have to extend as far as it did..