Blades/Killing Paarthrunax: The blades did help the dragonborn on his quest to defeat Alduin, but they didn't care about what the Dragonborn wanted. In my plathrough the Dragonborn trusted Paarthrunax and the Blades weren't willing to respect his wishes, therefor not truly serving the Dragonborn, but moreso his destiny through their methods. Greybeards/Sparing Paarthurnax: The Greybeards were the more reasonable faction. They summoned the Dragonborn to High Hrothgar and tought him/her the power of the Thuum. Before teaching him/her everything, he/she had to go through a trial, which is reasonable imo. Paarthrunax commited serious crimes to humanity, but I feel like he's redeemed himself. I feel like if Paarthurnax truly was evil, he would've risen to power as soon as Alduin was sent through time. Unlike the Greybeards and the Blades, Paarthurnax never hid anything from the Dragonborn, being completely honest with him. Long story short, I can't kill Paarthurnax without feeling extremely guilty.