My daughter is a staunch libertarian and hates the draconian, fascist restricts of most of the UK. Yet she LOVED London, and will attend university, in all likelihood, in Northern Ireland, as you know.
My point - and probably your point - is this: For people wanting to come and engage in learning the culture, food, history - for wanting to spend time touching historic sites, monuments, and buildings - Europe is an incredible destination.
Some of the realities I see: Isis is not blowing up traffic jams - absolutely vulnerable folks. They are not walking around our malls detonating bombs. They are not accosting people with any regularity. But shit happens.
Last september in Milan I walked out of a mall area near the main cathedral. One of the public trash bins was smoking as if something was burning inside. As I passed i stuck my head over but could see nothing. I moved my daughter and friend quite rapidly away - walking as fast as we could without looking like we were in a panic. Nothing came of the smoking bin, but we were right to un-ass the area asap.
My daughter walking through Amsterdam passed a woman wearing much-much too heavy of a coat for the summer. The woman had a large handbag and she was scuttling along staring at the ground. My daughter and her friend turned right down a side street and then left to put a city block of buildings between her and the woman. Nobody panicked and nobody was truly scared - but she did that simply because her spidey-sense was tingling. Nothing came of it.
When I have guests we travel mid-week as much as we can, and we lay-low on weekends if we can. We see sites at the extreme hours - either right at opening or end of day - but we try to avoid mid-day rushes. That's partly because i hate crowds
So - I'm trying to say, if somebody comes to Germany, specifically, they have to fear nothing more than their own fear. Common-sense awareness is the only measure required to ensure a safe trip. But that is not dissimilar to probably every trip to any destination.