1.) A weak argument. Until then and to that point is it trapped at this one place.
I also doubt it can really convince all the dragons and Paarthurnax did not convince anyone in the past either nor do the other dragons seem to have much intelligence. You really just got a fairytale ending ala Hollywood for the weak-minded believing it will all work out if you only let it go.
2.) There is no place for dragons in a world that once was and is again being terrorized by dragons. People will fear it like any other dragon.
3.) You really need to ask? It killed many humans for no good reason. It then was not some bystander, but it was Alduin's right hand. And an old saying says, you walk with them and you shall hang with them. Even if it had not killed anyone would it still be an accessory to mass murder.
4.) It did not do much with its life in all those years sitting up on the mountain but to fight its inner demons.
It was mercy to kill it and justice for all of Skyrim.
Paarthurnax managed to charm you and you fell for it. Even "little red riding hood" could see through the big bad wolf... Where does this leave you?
1. Paarthuraxx is a he firstly. Not an It. Now 2ndly, do the MQ w/o killing him! He does leave the mountain and he can't convince them all but he can sure convince a lot of them since the majority of dragons don't like Alduin that much anymore. Look at Odahviing as an example! Also, the dragons are quite intelligent, don't say they aren't.
2. There is always a place for another race, like a dragon. You are pretty much saying "There is no place in the USA for Mexicans!" You are racist!
3. He was a bad guy but he did turned good and he is good. There is no reason to kill him. Ever heard of "Forgive and Forget"?
4. He did a lot. He was by far one of the main characters within the lore of the Elder Scrolls. Without Paarthuraxx, Alduin could've won all those years ago.
Where it leave me? Where does it leave you? You listened to crazy old man and a crazy woman who are too paranoid that the Thalmor is doing this or other paranoid-crap that is making them crazy and unable to choose if someone should live/die. You tell me? Where does that leaves you? Oh and btw, the only crazy guy you should listen is Sheogorath.
Anything else you like to add? This is such fun for me!