If they go to fight for ISIS, or even help ISIS, they have broken the law and should be identified, put on trial and locked up. I would rather this than them committing atrocities in other countries.
Sounds great but often there isn't the evidence beyond going to Syria and joining the cult. They could be beheading people and throwing others off building, but when they come back to the UK they'll say they were just the cook. They'll get 3 years, cut in half for good behaviour and then they'll be out in our community, forming part of the Trojan Horse that is trying to kill our children.