If someone from your country joined ISIS and moved to Syria, should they still be allowed to return home someday?

Absolutes don't actually make sense, I'd say "depends on the circumstances". So-called brainwashing is a real thing, especially with religion (Maybe need more neutral term than brainwashing tho'). But then you have a problem, as long as they don't denounce violent beliefs, but this will be impossible as long as they're treated as an enemy. Odds are your country already has many people who have never left the country that hold even more violent views without anybody keeping an eye on them though.