I have been told it's bad etiquette to ask to see the villain's card in this spot.
Though I do not think it's bad etiquette.
If the villain mucks his hand already, how I understand it, is their hand is dead, and cannot be accurately retrieved. Therefore you win the pot regardless, and do not get to see villain's cards.
Personally I always show my bluffing hand, and I will always ask to see their (bluffing) hand, unless they want to muck before I have shown my cards - then that is their choice imo.
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