Nope, we should kill them all. First off, you can't ever be sure they didn't participate in the acts of barbarity IS rejoices in. Secondly, radicals have been known for years to openly endorse lying as a means of maneuver so you can never trust that you aren't welcoming sleeper agents. Third, the european out briefs I've read from those who return indicate the deserters don't reject a caliphate or violence as a means to establish it but rather the particular ways IS has done business. This is important when considering reintegration because these guys still buy into the concept.... They just didn't like the way this one played out; so what's to stop them from returning later if/when appropriately inspired?
Finally, for years I've been listening to folks say we can't kill our way to victory. But we haven't tried, and frankly I think it's a valid TTP. All the COIN talk and reintegration and hearts and minds, etc. In the right time and place may be appropriate, but we can't leverage those tools to our advantage unless we're first in a position of strength; one attained by killing enough enemy to stress their organization. They've absorbed our attacks better than we would absorb similarly porportional inflictions. I think with these guys, we can and should kill our way to victory. Kill everyone who deserts and trap the rest in NE Syria and let Assad deal with it. This isn't an impossible problem, we just need the stomach to execute at higher levels, because the line guys are willing and able.