I killed him for two reasons, he is a traitor, he betrayed his kind and I don't trust those types for anything. Secondly I have this thing where the character I play is the same character from Morrowind, who has survived all this time due to him catching the corpus disease, and only curing the negative effects while keeping the immortality it provides and THAT character has a long history of working with The Blades and owes a lot to them. He sided with the Blades who he has been an ally with instead of a dragon (the enemy) he JUST met.