In the books – and I make no promises, because I have two more books to write, and I may have more surprises to reveal – the conclusion that the careful reader draws is that Joffrey was killed by the Queen of Thorns, using poison from Sansa’s hairnet, so that if anyone did think it was poison, then Sansa would be blamed for it. Sansa had certainly good reason for it.
The reason I bring this up is because that’s an interesting question of redemption. That’s more like killing Hitler. Does the Queen of Thorns need redemption? Did the Queen of Thorns kill Hitler, or did she murder a 13-year-old boy? Or both? She had good reasons to remove Joffrey. Is it a case where the end justifies the means? I don’t know. That’s what I want the reader or viewer to wrestle with, and to debate.