One thing I don't care for is whether or not Tiber Septim didn't mind him. I mean, the whole issue of that guy's life is confusing, convoluted, and contradictory that I'm at the point where I don't care anything about him. The developers have thrown too many damn varying stories about him that it's pointless to talk about him. In some cases, he sounds noble, in others he's a backstabbing jerk.
I agree that the lore is becoming convoluted on a lot of subjects.What can you do though?
It's been convoluted since Morrowind, intentionally. That's the way it's written.
I did not kill Paarthurnax. He acknowledged his wrongdoing and seems to me to have made the steps to redemption. That fact that I was given the mission to kill him by the specific two Blades I got it from is relevant:
Esbern has had recurring nightmares about Dragons for some time, and is far more suited to academic life than making decisions of someones life or death. Delphine was so desperate to have the Dragons be the Thalmor's work in a way that suggests she has serious PTSD from the war and therefore is probably not the best decision-maker either.
Add to that that the oath to kill Dragons has been defunct for at least 600 years by this point, since Tiber Septim bound several to his will. The Blades are desperately searching for a purpose after 200 years of having none, and the two that have found one are of debatable suitability to make that decision. So there you have them betraying the oath to serve the Dragonborn to allow for the oath that allows for violence.
Also, Paarthurnax had just saved my hide when I was stumbling blind atop the Throat of the World after learning Dragonrend. To answer that with murder would be neither something I would do in the real world nor would it have been in-character for who I was playing.