OK, I watched the beetle scene again. I did feel, as others did, that aside from some tidbits that highlighted teh Jaime / Tyrion relationship, it was tedious and kind of pointless. But, after seeing so many posters try to make sense of it in the context of the episodes, I thought I would watch it again. What I've come up with is that Tyrion is drawing a parallel between the simple Lannister and Tywin. He even says that when Tywin would go on and on about history and dynastic ambitions and the Targareyan (SP) conquest, he wouldn't hear dragon wings, but the noise that the beetle-smasher would make. He would dream about a seashore made of beetle bodies. Tyrion is the beetle that Tywin is smashing and he is talking about it now because just like with the simple Lannister, he doesn't know why it has to happen and what it's all for.
I mean, immediately, he knows that this is about "did he kill Joffrey" but this has always been really about Tywin and Tyrion's relationship and the idea that Tywin cares about family legacy and has been trying to find away to keep Tyrion away and apart from that legacy since his birth. And no one will ever tell him why. So, that's my take on it. My other take is that it would have been better in the previous ep. It seemed an aggravating slow-down at that moment. Especially for book readers, who maybe aren't as much worried about Tyrion's fate. We were like Oberyn: we kinda wanted to make it about him, too.
I will say that D&D do a great job of not only faking out the Unsullied but the book readers also with the "oh! maybe they will change it and let him live!"