I don't believe in nationalism or patriotism. If a citizen defects to another country or organization opposing our country I'm not really outraged. Also, lots of people actually have good reasons for joining terrorist groups regardless of how evil terrorist groups are. Lots of Muslim young people join ISIS because ISIS promises them adventure, opportunity, and a dignified life with financial security which attracts lots of young people to their cause. They shouldn't lose their citizenship simply because they tried to find a better life for themselves. Every case should be judged on an individual basis. So no I don't think joining a terrorist group should automatically cost someone their citizenship.