I believe that no one is inherently good or evil. It is our thoughts and actions that are good or evil, actions being the more important of the two.
3,000 years ago, Paarthurnax committed many evil acts. Then he stopped, and has spent the past 3,000 years doing nothing but good, including doing everything in his power to right his past wrongs. There's no reason to believe that he poses any danger to innocent mortals, thus, to murder him would be an act of evil.
JM not-so-humble O
100% agreed. Also, when he did the 'evil' acts, it wasnt like he had a choice, was it?
if he, or indeed any dragon said "no, i'm not doing that" then Alduin and his buddies would either kill them on the spot, or ambush them later on.
I also dont see why Akatosh, who is a dragon, would create dragons and then turn around and say 'ok, you exist only to be killed...Sorry", as that doesn't make sense.
I for one have a easy answer for this debate: why not allow an option (or have it refered to in a future game) for both the Blades and Paarthunax andhis Greybeards to make amends, "Agree to disagree" and basically let go of the past, TRUST each other, and work together to help the world against and even more current and deadly threat...the Thalmor!
Seriously, Paarthunax did wrong, yes...but hasn't everyone?