Re: A gun is involved in every single mass shooting.
Then with respect, and since this is essentially a techno site, if I may I'll take your premise and apply a little logic.
1: It's fairly apparent that the US has a higher incidence of gun related violence and death than other countries (I'm sure there are those who would argue this, and they are of course entitled to. But I'm posing it as a premise).
2: Accepting your point (purely for the sake of logical review), let us assume the root cause is indeed not guns but 'evil, sick, depraved people'.
3: The logical combination of (1) and (2) would appear to suggest that the US has a significantly higher incidence of 'evil, sick, depraved people' than other countries (I'm not saying this is so, merely extending the logic).
4: Er - why? All those evil etc people in the US, I mean? What is the root cause under this logical structure?