Omg. Sorry for this super cynical viewpoint, but you are far more likely to be killed here in the good old USA by a lunatic with a semi automatic. Massacres of that nature are far less likely in Europe. You can do a little research by the way, because terrorism has been around for decades. I believe it was at its peak in the 70s.
I did study abroad when the IRA was putting bombs in trash cans all over London and the UK. I never felt in danger. As another person said, your odds are far worse everytime you get in a car. The omnipresent media distorts reality. I won’t stop going anywhere becasue of terrorism. Then they get what they want, and I won’t give it to them if I can help it.