No word for cousin killing - I tend to think that was a reference to Jaime killing Lancel. Jaime is the one who says "cousins". He looked like he was thinking back on his own kin-killing as Tyrion listed all the `cides. Tyrion may not understand why all the beetles had to die because in his world, he is a beetle. A dwarf who's own father and sister detest him. Even with all the Lannister gold and all the clever wit, Tyion is a beetle in the eyes of most men. I'll bet Jaime, a soldier, understands it a little though, after all he was kingsguard to the mad king Aerys. The reason the beetles had to die is because the world is fucked and those who can kill do kill. Because they can.