There was a lot of mess already with the current immigration crisis. Now add that attack, and like you said, it's a powder keg, except I'm afraid that this shooting may have just lit the fuse. People will be connecting those two issues. They already are, judging from things that start popping up on my Facebook feed.
I fear the overreaction coming, of both citizens and governments. I fear the Europe will split, or start a war with someone, or draconian security measures will be introduced by the government. Charlie Hebdo was symbolic. This was a real terror strike. I just hope that sanity prevails and we won't amplify the damage further.
This is exactly what the attackers want you to be. Don't let them win.
It goes away and you realize that the equilibrium in a civil society is normalcy, and to give in to fear is to lose. Recency bias, forgetting your math/probability/statistics--you're smarter than that. Lead the way for others to keep calm and carry on.