I suppose I am just generally in disagreement with the concept that anyone has to be responsible for enemies of their host society.
Exiling people who harm or oppose the community in a dangerous way has been a reasonable and accepted practice since forever. For that matter, I would love to exile our German fascist supporters to Russia so they can die for the führer they so idolize. These people are technically brainwashed too, victims of Russian disinformation campaigns. Does that absolve them from responsibility? No.
To return to the original example: If they want to join a religious terrorist group, alright, but then they are that group’s responsibility. If that group are just stateless, disorganized fanatics that couldn’t possibly provide a good way of life for anyone even if they had the resources, that’s not anyone’s problem but their own.
There are some things that are not forgivable in my opinion, one of them is to set out to actively participate in a religious terror campaign. Why should any other society be responsible for them?