Not terribly interested in arguing about the definitions-- I disagree with your definition, but it's not relevant to my complaint-- you get my point, they said they did it to motivate Moiraine.
I don't know if it counts as a hint. They've been playing fast-and-loose with the Oath against killing "unless they're a darkfriend, or in defense of her life or her warder's life or the life of another sister" this whole time. If we're taking that execution as a hint, then what do we call it when Moiraine took out half a Seanchan fleet in the last season?
I don't know that there was any more depth to the scene that I was giving it. They said they wanted to give Siuan a big speech and motivate Moiriane. That's what they did, and they cut a really time-consuming subplot. You could argue that it was a very practical choice.
But her getting dead impacts Moiriane immediately and obviously and for the Tower it mostly just cuts off the cruelty and the torture that Siuan's speech emphasized. All it tells us about the Tower is that they are down with torture (which, no shit, everyone in this series loves torture) and they are merciless to darkfriends (which, no shit, so is everyone in this series).