The system of integration for migrants failed in Sweden (as it was hardly even existent in the first place). That's the systemic cause.

The acute symptom of this currently is gang violence. That said, the symptom is like a cancer that runs itself, the criminal systems underpinning gang violence are now established and you won't get rid of them by being nice anymore.

So you have to fight both the symptoms and the causes... Which Sweden is beginning to do. Reinstating and hardening penalties for organized and violent crime to get rid of the acute symptoms, as well as stopping the inflow of non-European migrants to stop the cause.

The Swedish government didn't say they'll never take non-European migrants again, but it's clear the whole system needs a huge overhaul.