Besides, both Dragons are evil.
Firkragg, Gorion( your foster father) himself tried to kill Firkragg, so there has to be a reason why. Firkragg kidnapped Garren's child( even though Garren posed no threat to him, and was merely a lure to get you to come) and financially ruined him. Firkragg also threw away the lives of several Paladins by casting an illusion over your party and them. Furthermore, he hoards an artifact that belongs to the Paladins.
And you ask why we want to kill him?
The Shadow Dragon( ain't gonna type out the whole name) is an abomination, and a threat to the world. The only reason you can bypass the fight is through a wardstone, but you will be doing Faerun a service by getting rid of it.
There are five Dragons in the game. Three of them are evil. Good-aligned players like myself will not fight the other two.