I think I partly agree with the idea of stripping someone of their nationality or citizenship, after doing something as stupid as joining ISIS (assuming that has been proven beyond any doubt), especially if the person is clinging to their nationality as a way of demanding certain benefits and avoid harsher penalties.

I see your point about the intrinsic value of nationalities, but I don't see the measure empowering ISIS.
I was rather thinking about the risks of extremism and the impending threat over innocent people who may be unjustly accused of high treason and terrorism and their whole families being stripped of their rights for political, religious or moral reasons. It would set a very dangerous precedent.
I thought about the reason families disown their kids, usually the girls, when they dishonor their name. They don't want to be associated with whatever wrong their kids did. That is a way of avoiding responsibilities for those wrongs. Politicians tend to do that, but States shouldn't.