"What is better - to be born good, or to overcome your evil nature through great effort?"
Esbern told me that I must kill Paarthunax. I joined the blades rather than the Greybeard because I thought them to be too 'culty'. Turns out the blades are no different. When you played, did you kill the dragon or not, and why?
I can't kill Paarthurnax, he helped me with his advice, he helped me with my shouts, he's teaching the dragons how to be peacefull. Whatever he may have done in the past he also helped to stop Alduin back then. Killing him for the Blades is just doing evil, and it's not as if the blades have much to offer.
I let him live. The Blades are too caught up in old scores and aren't looking at how to solve the problem in front of them. My character can kill any dragon I fight but I can't kill every dragon. As evidenced by the fact that they still randomly spawn after I killed Alduin.
Paarthunax admits that he won't be able to convert all the dragons to his teachings but he wants a chance to teach dragons that they can live in peace with mortals. So my character has a simple deal with Paarthunax. I won't go chasing after every dragon I see. I will give any dragon who is willing to heed his teachings a chance to live in peace. But if that red dot pops up on my compass, they're mine. And fitting that I use Dragonrend as soon as they go on the attack.
On a side note, it seemed fitting for my character to pick a fight with a group of people who are sworn to serve me. I really couldn't end the main quest on a better note than that.
I spared him in my only MQ playthrough - I may make another character in the future that will kill him. I will have to make a good backstory so it does not seem dishonorable, or create a character that isn't concerned with such things, or perhaps simply a 'dragon hunter'. Emer's Tsaesci Build is an interesting choice, it gives role-play reasons for Paarthunax's death.
Question: Can you kill him when you first encounter him? And if yes, can you continue the MQ without him?