No, they should be detained, investigated, and given a fair trial once they re-enter our country

If you, by your own will, have chosen to join and carry out illegal activities for a terrorist organization, you have made your choice. Deal with the consequences of your own actions. You don't deserve citizenship of this beautiful country. If you want to try prove you are no longer affiliated then, fair trial and investigation required with regular checkups herein afterwards with citizenship still revoked.