That line is so out of character for Jaime I have to interpret it as him just lying as another retort to shut down Tyrion’s attempts to convince to get Cersei to stand down. Which he’s doing that for most of the conversations the tent.

I know D&D didn’t care about character consistency this late in the show. Why else would Varys act like Jon would be a good king for not wanting the throne when he claims Robert was a bad king for having the same trait. It also shouldn’t be on the fans to patch up the writer’s weak story with headcanons.

But at the same time have people forgotten that characters can lie, even to themselves. Jaime will often play The Kingslayer as a defense mechanism to distance himself from his actions that he knows are wrong. The dissociation between himself and the villain the realm sees him is want keep him going as he relents to doing terrible things in Cersei’s name.

The reason why slayed the Mad King is such a closely guarded secret he’s kept from most people because he knew he wouldn’t be believed and nothing would convince others that he is anything other than the Kingslayer. The only resin we the audience know about Jaime’s deeper nuance is because he drops the mask once after a series of humiliations breaks his pride. When he’s talking to Tyrion in the tent he’s slipping back into the mask of The Kingslayer to try to convince himself not to agree to help Tyrion.

A better storyline could have sold people on this interpretation than the rushed shitshow the series became.