I can't kill him. No member of the Blades tells me what to do. Delphine thinks she controls the Dragonborn, pfft.
I couldn't really vote for any of those options. Yes, he committed atrocities in the past, but he was key to helping save the world. I personally feel this has earned him a second chance, and I let him live, but I'll keep an eye on him. If he does start to give into his nature, then I'll be forced to destroy him. Not before, like Delphene wants.
http://i.imgur.com/FvjkJ4h.png
This is basically what happened to that thread
In my book, redemption mandates both regret and atonement (neither of which applies to Parthanaax). Hiding out on a mountain doesn't qualify for redemption, and Parthy himself admits the ancient Tongues didn't trust him enough to let him in on their plans. I also didn't fail to notice how lukewarm he was on taking action against Alduin, and didn't appreciate his lack of forthrightness.
I don't attack any dragon that isn't attacking me. My Dovahkiin characters think of them as kinfolk. Delphine calls my DB character "the ultimate dragonslayer", but as I see it, my character appeared on Tamriel as a DB for the purpose of casting Alduin back into Aetherius to keep the kalpa from prematurely ending; not to commit a genocide. Akatosh be praised.
So yeah, I view it as fratricide. Paarthurnax helps the DB accomplish what is ultimately Akatosh's desire - that the kalpa end at some other time than 4E 201 or thereabouts. That's a pretty favorable outcome for everyone and everything. The Blades' ongoing grudge just seems so petty and small in comparison.
I installed Arthmoor's http://skyrim.nexusmods.com/mods/18465/? which apparently lets you give Delphine and Esbern a stern talking to - but I haven't seen it in action yet. Haven't reached that part of the MQ.