I wonder how big an impact the free flow of guns in Americas has had on the violence in the continent. It seems like one of the main reasons behind the extreme Mexican violence, at least, is the uncontrolled US weapons market.
There have been lots of violent and terrorist movements from the 70s onward and even prior to that. Guns didn't help but the US is far from the only suppliers or cause.
As for the US weapons flow to Mexico, there was a NPR article last year on the topic [1]. Wikipedia [2] mentions "many of the traceable weapons come from American weapons markets and festivals that do not have regulations for the buyers", which very much is the uncontrolled weapons market, right?
To be fair, the Wikipedia page also points to the other direction with "weapons [..] come from either government personnel who defect to the cartels, or are sourced from Central American black markets".
[1] http://www.npr.org/sections/thetwo-way/2016/01/12/462781469/...
[2] https://en.wikipedia.org/wiki/Mexican_Drug_War#Gun_origins
If the cartel is filling the void (granted the things they enforce are different) then you may as well make it an equal comparison.
I really wonder how the US graph would look if you included people killed by police.
Gangs murder people out of cold blood.
The overwhelming majority of police killings involve people who present an immediate danger to the innocent and are violent, dangerous criminals.
Washington Post has a neat interactive page I came across. The most shocking thing is the unashamed misandry when it comes to police killings. This year, only 25 women were killed versus 556 men. That's rather ... disproportional.
https://ucr.fbi.gov/crime-in-the-u.s/2014/crime-in-the-u.s.-...
http://www.killedbypolice.net/kbp2014
https://www.washingtonpost.com/graphics/national/police-shoo...
Last year the US police killed 1162 persons, so that's a rate of 0.36 per 100k inhabitants.
The Mexican cartels control politics anyway, so good luck just making them go away.