Imho, they haven't built the betrayal as hard as they did in the books. In television world they had it as Shae being niave and feeling Tyrion betrayed her first (kicking her out, calling her a whore). Shae in the books was a much harder edged character and it would play out differently for Tyrion to murder her with his bare hands in a rage.

Crossbow arrow I think is a nice compromise, lol