Canada has numerous treaties in place that can see the perpetrators of a crime returned to the jurisdiction in which they committed (or allegedly committed) a crime.
Any nation can easily make these wannabe GI Joe's think twice about going on a shootem-up vacation if they simply grab them, put them on a plane and ship them back to where they will have to deal with the consequences.
... And yes, I understand that there will be all kinds of lawsuits, but let them fight those suits from Syria or Iraq whilst they answer for their killing sprees in those nations
Exactly. The foreign state would first have to gather any evidence so that probable cause can be established, then it has to submit it to Canada which, in turn, would have to determine that the suspect should be detained and returned to that foreign country for trial. That is very difficult to do especially since these characters wear masks and facial garb when in combat situations. A good lawyer would have an easy time making a case for why someone should not be exiled under those circumstances.