"Last Stand" was written as a worst-case-scenario blocking card, but in development we were also aware that the card can be used as a kill card. When combined with the card 'Startle", this interaction is powerful, repeatable, with very little room for interaction. After "Last Stand's" powerful presence in the championship decks, I think it is time to clarify the rules on this card.
The keyword "Fades" was introduced after "Last Stand's" printing in Clash of the Battle-Goats. I believe that if that keyword existed in Clash the card would have read "When this card Fades kill that gruff" rather than the much more vague language "When this card leaves play, kill that gruff."
As of today the official ruling at all official tournaments will be that the card should be interpreted as if it was printed with the card as shown below. I believe this reading on the rules keeps "Last Stand" interesting and powerful, but still leaves room for interesting interactions and counter-play. Feel free to print this image as a sleeve insert in your decks.
(Design Director, Studio Woe, LLC)