Think You Answered Your Own Question There...

Once one of the most peaceful countries in Europe, Sweden is plagued by the worst rates of deadly gun violence in Europe, 10 times higher than Germany.

...

As Sweden searches for solutions to its surge in gangland violence, an obvious question emerges: how did one of the world’s most open societies incubate such a vicious subculture?

Gosh, I don't know! Are there any clues? 

Hjällbo’s young gang recruits exist in an environment of exclusion; failing schools, joblessness and poverty. Integration into mainstream Swedish society appears minimal at best.

Hjällbo’s 9,000 residents are almost all immigrants: nearly three-quarters of them were born abroad.

Well, well, well... 

Hjällbo is the turf of the Ali Khan criminal network, Gothenburg’s most ruthless gang. Originating in Lebanon, the clan has fostered a reputation for retaliation. Murder, extortion and extreme violence are linked to family members.

Looks like diversity isn't a strangth after all, eh? But of course, to put the icing on the cake, you also need lax laws biased in favour of those dedicated to breaking them. And would you believe it, Sweden also has those in abundance: 

Sweden’s famed openness still presents barriers to policing. Issues such as privacy override calls for increased investigative powers. Nord’s detectives are denied ready access to the CCTV network, along with ANPR (automatic number-plate recognition) technology and even access to traffic cameras. Only recently have they been able to tap phones.

It's a perfect storm; floods of people from a strange land with a lawbreaking culture in a country ripe for ravaging.