No common element in US attacks? I think in the majority of cases that's just not true. People weren't killed, but the PizzaGate shooter was motivated by what can only be described as a large cult of a particular persuasion, Planned Parenthood shooter certainly had his political motives, Charleston shooter had motives that have existed in the US almost continuously, Oregon standoff were all anti-government types, I could go on and on. I would say the main difference is we are not actively at war with people that share their faith or look like them overseas. As far as I'm aware the Boston bombers and Orlando shooter were only inspired by, but received no support from groups such as ISIS. In that way they were just as self-radicalized as Dylan Roof the Charleston shooter.

Obviously it's all more complicated than I could quickly write, but to me there's a big difference between the self-radicalized generally disconnected persons in the US or U.K. versus those on the ground overseas.