There is another reason FOR killing him that stands if you are more savvy to the nature of TES.
"Mantling", the best summary I'd seem goes something along the lines of "To walk like them until you become them". it seems odd to us, but in the TES universe when it comes to spirits, if something looks like a duck, walks like a duck and quacks like a duck, it becomes a duck. You've already seen it happen once - the Champion of Cyrodiil walked the path of Sheogorath until they
were Sheogorath.
Now, let's look at paarthunax...
- He's a dragon, like Alduin - Check
- He wants to lead the other dragons like Alduin - Check
- He has a human cult who will obey his command (Greybeards), like Alduin did (The dragon priests) - Check
- He has an intrest in humans and dragons working together (like Alduin once ENFORCED) - Check
And there may be other details I've overlooked, but the key thing is Paarthunax has already begun to walk a path that is superficially close to that of Alduin. The Champion of Cyrodiil's path may have differed greatly from that of Sheogorath in the early days of his career (During Oblivion), but 200 years down the line, the CoC and Sheogorath are indstuinguishable, he has been mantled. In 200 years time might Paarthunax not have simply replaced Alduin? - The world is
meant to be eaten, it
needs it's Alduin, and there is an Alduin shaped hole waitting to be filled...
Either way i havn't killed Paarthunax personally, but from a lore perspective there is a very real danger posed by his existance if you are savvy with the physics of the setting
Problem. Here some reason as to it:
--He's a dragon, like AlduinHe is X speice, just like his older brother. Therefore, he is bad. Thats like saying that Meridia is evil because she is a Daedra Lords or the Underking is evil because he is undead/lich/whatever the hell he is. Beside, the Dragons serve the Empire right after Alduin defeat.
--He wants to lead the other dragons like Alduin - CheckAnd Ulfric Stormcloak want to lead the Nords like their ancestor to free themselves from the Empire to worship Talos. Or Gortwog gro-Nagorm want to lead the Orsimers to be equal in every race eyes. Just because they lead a group of the same race doesn't always mean bad things will occur.
--
He has a human cult who will obey his command (Greybeards), like Alduin did (The dragon priests) - CheckEvery Daedra Lords, every Divines, every Local Gods like the Tribunal have worshippers like that. Does that make them all bad?
--He has an intrest in humans and dragons working together (like Alduin once ENFORCED) - CheckSo did the Empire and the dragons gotten riches and security in exchange for military service.
There is more then one way to end the world, Alduin just so happens to being one of them. The other way is destroying all towers that hold Mundus together, which is the plan Thalmor wanted if they ever want to be gods themselves again.
Beside, if Paarthunax wanted to see the world end, he would have not waited thousands of year to help the PC out and betray his own brother. IF anything, the end of the world would happen back then and we would not be in this kalpa right now.
The problem with the reasoning that you are the last dragonborn is that people don't need to be dragonborn to kill dragons.
If the dragonborn kills all the rampaging dragons, you still have the blades order after he dies. If Paar reverts to his old ways, there'll be another great dragon hunt until there are no more rampaging dragons again.
Hell I should be killing everyone in Nirn because they might possibly become a bandit someday. Rikke, Ulfric, Galmar, and Tullius too. They killed lots of people in the great war, so they deserve to die.
Indeed. Without Alduin, dragon cannot be resurrected again and the extinct of the race can occur.