I can think of a much quicker method that doesn't involve messy trials and it is a permanent solution.
---I can think of a much quicker method---
Yes, but you can't kill that many people in Western Europe nowadays, without throwing the whole facade of western democracy out the window and no one is willing to do that, for now. It seems these thugs can't simply be deported either. So you make a decision just as we did with our inner city problems and you start budgeting for jails to hold a lot of people.