The Harry Hugging Gang
While many dragonborn worship Bahamut, Klaa’s family was devoted to Tiamat. While not forbidden (in general, dragonborn only care that you choose a side), the preference certainly wasn’t a popular one, and Klaa’s family worshipped in secret in order to avoid any negative social standing.
Klaa himself wasn’t particular devout. In fact, he’d probably have fit in better with a traditional, Bahamut-worshipping family. He was gregarious, had interests in medicine, and was known for both his good advice and hearty back-slaps. All around, a good dragon-ish guy.
Shortly before Klaa would have left home, his family was struck by disaster. A particularly zealous sect of paladins who worshipped the Platinum Dragon discovered the family’s hidden temple, and attacked during a holiday ceremony. The entire site was destroyed, and Klaa’s relatives were slaughtered. Klaa himself only escaped because he was able to dig his way free of the rubble.
Tiamat is the god of vengeance. And when Klaa stood saw what had been done to his family, he swore allegiance to her on the spot, and dedicated his life to eradicating any and all of Bahamut’s followers, even Bahamut himself. The Dragon Queen accepted his offer.