Apparently, armed citizens can help in some cases. Three armed people stopped a nuthead shooting in train in France:
http://www.theguardian.com/world/2015/aug/21/amsterdam-paris...
http://www.theguardian.com/world/2015/aug/21/amsterdam-paris...
My feeling (being a European) is that I would feel much more threatened by the fact than people I can see in the street can have a gun than I would feel safer because some random strangers could protect me with their guns.
From time to time there are some news that someone get stabbed because for having allegedly had a bad look on someone else. With guns, you don't even have the option to run.
There are civilians lawfully carrying guns in Europe, just not many of them, so be free to already feel threatened. I'm not saying we should give guns to every kid out there, but if 1% of citizens were armed, shooting up 100 people wouldn't be such an easy endeavor.