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but its fine for them to do what they like to people without impunity and without trial?
Er, no. Straw man alert.
Chopping people's heads off and engaging in acts of terrorism is against the law already. Removing people's nationality will not make it any more against the law. Preventing a state from removing their nationality will not make it any less against the law. Not removing someone's nationality does not mean that they are free to act with impunity and without trial, and so the fundamental premise of your rhetorical question is flawed to the point of non-existence.
Not if you value law. If you take someone's nationality you put them in a legal limbo where you can do what you like to them with impunity and without trial. And that's all that separates terrorists from the rest of us.