You gotta think bigger. Death would be too easy for the dragons. Shatter their pride and drag it through the mud. Cut off their wings, seal their magic, and then force them to work in the restaurant serving humans and other races.
A bunch of red dragons and black dragons all pooling their powers together and they haven't even scratched him. Surely even these idiots can get a clue, right?
With the arrogance of a dragon and the ignorance of a young gun, I'm not surprised they never gotten a clue. The way they constantly refer to him as "just a human" and not the "human that slayed dragons" tells you all you need to know about how they are and what they think they can get away with.