Is it normal i think we should let is sympathisers leave for syria

I don't understand why countries with lots of IS candidates like France or Belgium don't just let them go. It's less up-to-no goods and potential thugs to fear locally and who will anyhow harm us at some point or the other. If we let those motherfuckers leave at least we can bomb them over there. Those who return should immediately be shot in the neck, execution style. Stop the leftist madness and mollycoddling already.