Running HotDQ.

My PC Dragonborn Druid's backstory is great. His twin brother disappeared from his tribe one day while the PC was away training. He left the reason up to me. In a flashback scene, I revealed a conversation the missing brother had with their dad. The Dad said he 'knew he had to go to with them to save the family, it was the only way'. The brother agreed. I left the reasons unclear at that time.

So.....ideas why a Dragonborn future tribe leader would abandon his tribe and work with the Cult of the Dragon that wouldn't make him evil? I want to reintroduce the brother at some point in the campaign.

Thanks for any and all ideas!