You're right, making a direct correlation between murder/violent crime based solely upon gun laws is myopic. I would argue that if Canada, Australia, the UK, Germany, etc greatly relaxed their gun laws you would see an incremental rise in violent crime and murder, but certainly not anywhere on the level that you see in the USA. If guns represent a can of gasoline, harmless on its own, then the almost total lack of social infrastructure in the US represents the lit match. Unemployment insurance, welfare, health care, child care, education, sex education, "right to work" states, abortion rights, drug laws, and the US prison system all represent an egregious lack of safety net for the "unfortunate", whereas the aforementioned countries invest heavily in these social systems. No matter what their life circumstances may be they've always got the base necessities-- quality education for their kids and health care for the family. Should you lose your job, your unemployment insurance isn't a total joke, and if you get arrested for drugs, the focus is on rehab and not incarceration. If you are a 15 year old girl, you can get BC pills on your own, and if you get knocked up, you don't have to worry about having to cross a human chain to get into the abortion clinic. So why does this all matter? Well, for one not having access to basic necessities makes one desperate, and the lack of education and the prison system seals the casket by providing no "out" to ones lowly existence.
A good example was a guy who lived a few apartments down from me when I lived in Canada. He was your quintessential lazy deadbeat and had admittedly never held a job for more than a year. He had been on welfare for 5 years and did the occasional odd job painting fences/garages to make extra under the table cash, but basically did nothing all day but drink beer and hang out in his studio apartment watching TV. He made enough money each month to pay the rent, cable, some beer/food, and pay for clothes at the Salvation Army. He had a kid and a poor ex-wife which of course he could contribute nothing to financially, yet his kid still got to go to a good school and have his healthcare/dentist fully covered. The guy literally didn't even WANT a job, he was perfectly happy living out his meager existence as it was. He had a place to sleep, had his TV, and had his cholesterol meds paid for by the govt. At the time I hated this man, because every time I came home from a hard day's work he was drinking a Sleeman's Lager on his balcony, and I would think how my tax dollars were floating a lazy asshole like this. Basically, this guy was a sterling example of Republican worries/mantras of "socialism", "not on my money", and "get out and work". BTW, his son despite having poor parents and being an average student, was still able to make something of himself and attend university, as the position he was born into had no bearing on his financial ability to gain an education.
Now, what if the person above lived in the US? It's no longer a question of "well I'm given enough to afford shit food and a place to sleep", but "I have now hit rock bottom- I literally have nothing to lose and am entirely despondent and desperate; knocking over that liquor store is starting to look like my only way out".
I remember after I first moved to the US one of my buddies asked me what it was like, and I responded: "It's like the wild west-- every man for himself. Let the strong survive and the weak die. Attitude seems to be as long as I live behind my safe golden gated community, I don't really give a shit what happens to others. From sub-prime mortgages preying on the dumb and ambulance chasers advertising on billboards-- it just seems so "caveat emptor" compared to home." But you know what, initially I LOVED that philosophy. No longer did I have to pay for that deadbeat neighbor with my tax money, now in my new home he can either get off his ass and work, or starve..
But as time went on, I began to realize that there is A LOT more to it then work ethic and tax money well spent. I saw outdated public schools that look more like prisons than places of education, "poor-ish" areas that exuded more rot and crime than even the worst neighborhood back home. The amount of "poor white trash" and the generally uneducated compared to back home was also staggering. Let's throw in a teen pregnancy epidemic and a shit backwards drug policy that incarcerates simple users and you've created not only the ideal environment for crime, but a merry-go round prison system that ensures that if you weren't a hardened criminal going in, you certainly will be one on your way out.
Guns are 50% of the US's problem, the other 50% is a failed social policy. Now there is NO doubt that if all handguns and semi-automatic weapons went *poof* overnight in the US that there would be a reduction in violent crime-- this is irrefutable. However, the amount of incremental crime as a result of just the "gun" being considered a factor pales in comparison to the synergistic effect that occurs when you place them into the hands of millions of poor, uneducated, despondent, disenfranchised US citizens.
Yeah, you could solve a lot of the problem by removing all firearms, but not only is that unrealistic at this point, but society would benefit far more from correcting a failed social policy that would not only impact violent gun crime, but myriad other issues as well.