"They hate our freedom", "Guns don't kill people, people kill people" and the ever trusty chants of "Lock 'em up and throw away the key/bring back military service/reintroduce the death penalty"as a solution to all of society's ills.
I've lost count of the number of times I've read pointless, rambling internet discussions where opposing groups of people hurl brainless catchphrases at each other like shit-flinging chimps.
So if it's easy answers that people want, how about this?
What makes ordinary people with grudges against society lose the plot and spray schoolchildren with machine guns? Why would educated British men strap bombs to themselves and detonate them on crowded trains? Why do terrorists commit nihilistic murders with no ideological purpose?
If I had to reduce it to one single cause that an eight-year-old could understand*, this would be it.
*The comprehension of an article by an eight-year-old is, incidentally, the standard test for publication in a British tabloid.