There is a German website that carries some statistics and these statistics show that it's not really the weapon laws or the population density that leads to a higher homicide rate.
http://www.fwr.de/zahlen und fakten.htm
The first table is: US-State | Adults owning weapons | Homicide rate per 100,000 inhabitants.
The next one is: Country | Households with weapons | Homicides with guns per 100,000 inhabitants.
The site also says that e.g. in Vermont there are more weapons, and more liberal weapon laws, but less killings compared to New York.
Also, countries like Germany or England have very strict weapon laws, but also a higher rate of illegal weapons among the people.
It seems a bit strange, but the study quoted on that site, Prof Lott, University of Illinois, More Guns, Less Crime 1998, found out that states that allow the concealed carrying usually have crimes, and the table shows the estimated reduction in crimes if the concealed carrying of weapons was allowed all over the US:
Homicide: -1,400
Rape: -3,700
robbery: -11,000
aggravated assault: -60,000
But an article from Amnesty International stated that in Australia that rate of crimes with guns decreased by 45 per cent after weapon laws became more strict.
I think a big problem is social security.
If you are facing social problems, like unemployment or some expensive operation, and you can't rely on the help of your state, and your future prospects are bad, you're more likely to become a criminal than if you get some support from your state or society.
Also, the problem with gangs that kill each other is more common in the US,
I've never heard of any gang fight here in Germany or another European country, with the exception of the "war" between the Hell's Angels and Bandidos that took place in Denmark some years ago.
Our "FBI", the Bundeskriminalamt, says there were 413 homicides in 2005 with 82.4 million inhabitants. 119 times weapons were at least present, but not always used.
But we also have a lot of cases a homicide isn't discovered (no victim or seen as accident) or is decided to be manslaughter, bodily harm with fatal consequences or something else. 95.8% of cases were solved.