Not when the cause they are joining is well documented as fomenting international terrorism and committing mass murder in the areas they control. 
Since when is it acceptable for your "citizens of conscience" to join a cause that summarily beheads prisoners en mass and conducts the genocidal slaughter and sexual enslavement of an entire ethnic group (Yazidis)?

Our troops are currently aiding in that fight, as part of a legitimate multi-national operation, at the request and invitation of the local government. Should these people ever return, they should expect to be taken into custody and face very serious criminal charges with a suitably long period of incarceration for being part of a recognized terrorist organization and engaging in armed conflict against Canada. They would also need to be investigated in detail for any culpability in specific cases of murder, torture and slavery.