This 1 line of 'Game of Thrones' dialogue could have a huge fallout for this character

Warning: Contains spoilers for Game of Thrones Season 7, Episode 3.

Lady Olenna said a lot of things about Cersei before she died. None of them were particularly complimentary.

If there's one thing that will go further towards shaking Jamie Lannister's resolve than anything else, though, it was her final words.

First, though, let's quickly recap.

After the Lannister army had marched on Highgarden and beaten the weedy Tyrell forces, Jamie went to visit Lady Olenna in her room. And before she drank the poison he gave to her, she gave him a few home truths.

"Whatever I imagined necessary for the safety of House Tyrell, I did," says Olenna. "But your sister has done things I wasn't capable of imagining.

"That was my prize mistake. A failure of imagination. She's a monster, you do know that?"

We've known since the end of Season 6 that Jamie is at least partly aware of just how deeply unpleasant his sister/lover truly is, but in this scene he starts out by defending her.

Then Lady Olenna casually reveals that it was her -- not Tyrion -- that actually poisoned Joffrey, and drops this final line:

Original image has been replaced. Credit: Mashable

So why is this important?

It's pretty obvious that Jamie is going to turn his back on Cersei at some point. We know this. They've been rifting for a long time now, and the signs have been there for awhile.

For whatever reason, though, he's still clinging on to the hope that she may change. That after she wins the war, things will get better.

Now, though, he has confirmation that her reckless actions have led to the deaths of not one, but two of their children. Tommen's decision to leap to his death was the result of Cersei murdering Margaery, and Joffrey's poisoning was the result of Lady Olenna's hatred/fear of the Lannister dynasty -- of which Cersei almost certainly played a huge role (don't forget she threatened to have Margaery strangled in her sleep).

That expression on Jamie's face as he leaves the scene isn't just anger and shock at hearing how his son really died -- it's also the dawning realization that his sister will never, ever be a safe person to be allies with.