^So your moving onto crime rate, that's ok.
So the UK might have a higher crime rate per capita than the US. Yet even with a higher crime rate per capitia it still has a lower homicide rate per capita.
Something like 70% of the homicides in the US are gun related, compared to something like 10% in the UK.
So using the 2010 figures for the US and the 2011 figures for UK, the US was something like 13 000 (9000 firearm), the UK something like 650 (60 firearm related 300 or so accouting for population size difference). Which means something like 3000 murders in the US weren't firearm related, Whilst in the UK it was 580 as the UK population is roughly one fifth of the US factoring into popultion that would equate to ~2900 or roughly the same as the US.
So tighter gun laws tend to equal fewer gun related deaths. Of course there are exceptions.
And lets be clear having strict gun laws doesn't mean that this type of mass murder can never happen again, it's all about minimising the chance of it occuring again.
What if it was your child that was brutally murdered?
Also countries might record crime in a different way, which might skew the figures somewhat.
And of course other factors can be involved, no one is denying that. But you have to tackle as many issues as you can. In order to prevent tradegies like this. So that might include mental illness, but that doesn't mean you can't also look at reviewing the gun laws.