It's not. The sin was Euron Greyjoy not killing Cersei, Arya not killing Daenerys, and THEN Jaime killing Euron. The writers maybe were distracted by the spinoffs, so they screwed the pride they could have had by finishing the JOB!
And Melissande dying was not enough for me to accept Daenerys would invite all of King's Landing to a massive cookout. That was perhaps the worst sin.
edit: One blessing, I loved how Tyrion set up a fledgling House of Lords and began a new system of selecting a monarchy. That was brilliant.