When innocence is no security, there is no incentive to be innocent.
Further, I believe that not arresting literally everyone who comes back and locking them away will actually decrease the amount of terrorism. It's called complementary behavior. Yes, some people will still end up being able to get away with it. That's why I think they should be monitored, spoke to and helped to have a place in society.
Some will still be fanatics, but the proportion of people who are true fanatics versus people who turn to crime and holy war as a way to try and feel meaning in their lives and strike back against a society they feel has abandoned them (regardless of whether it's true; if your family car gets vandalized multiple times a month and ethnic slurs get carved into your door, and you're a dumbass teenager with no sense of scale and hormones raging, you can make some incredibly bad decisions) is, as far as I can tell, a lot smaller than some people think.
But I think that locking up everyone who comes back will contribute to that sense of exclusion, frustration, and resentment. No, we don't want to 'just live with terrorism', and I think more than one approach will be necessary, but honestly, look at Aarhus. They took up a similar strategy, and the number of people leaving each year to go to Syria dropped from around 30 in 2013 to 1 in 2014. Doesn't work perfectly, but preventative measures, overall, tend to work better than reactionary measures for most things. Health, crime, whatever. Easier to prevent a problem than solve it.
I do agree with you on the crime thing save that rather than expanding death penalty, I feel like social programs and rehabilitation should be expanded to prevent shit from happening in first place. But again, this might be a matter of the differences between America and Europe, where America has imprisoned the largest portion of it's population of anywhere in the world, whilst still having fewer police officers per capita than most of Europe, like a third less, largely due to mandatory sentencing requirements and war on drugs.
This is europe thread, though, so I suppose I'll just ask: How common are repeat offenders? Because the thing I most remember about scandinavian justice system is that there was that breach in prison security when doors were left unlocked and the prisoners used it to bake a cake, because they didn't see a reason to try to escape or whatever.
@Martinuzz: Non-muslim people go to Syria to do volunteer work in refugee camps. Muslims can't? They would have no reason to see it as helping their fellow muslims? No way of thinking of that as a holy calling? Mujahideen doesn't just mean warrior, nor does Jihad (now) only mean war. Teenagers can, and often are, stupid. That includes running off to a foreign land because you think you can help people, as well as running off to a foreign land because you think you can 'defend the faith'. I don't think retributive justice, as in justice meant purely to punish and avenge, is something we should strive for. Meant to disincentivize, prevent, and stop behavior, sure. But locking everyone up, or killing them, gets real bad real fast. Cost, PR, and effectiveness all suffer.