Hi, I´m from South Spain.
The most criminal city in Spain without a doubt is Marbella, although theres no streetgangs there. Spain it´s the european country with the fastest growing number of organized criminal groups. And Marbella it´s the capital of this world. It´s plenty of eastern mafia: russians washing their money, lithuanian (who control mostly the prostitution), bulgarian and serbian (dedicated to high profile robberies), colombian (cocaine trafficking, distributing to Europe through Spain), Morocco mafias (trafficking with hasch and human beings), etc. Also, italian mafia (mostly napolitan Camorra) had their presence.
They do money laundering speculating with houses pricing (in the last seven years house price in Spain has risen more than 121%).
In Marbella for example the last two mayors ended up in jail. A few weeks ago, a police operation in Marbella arrested many lawyers and notaries for laundering 600 millions euros (around 770 million dollars) coming from many murders comitted in Russia.
The killings comitted there in Marbella use to be execution-style, although sometime it ends in a bullet hail. Early this year, three masked men tried to kill an algerian snitch related to hasch trafficking. They start firing at him with automatic weapons in a commercial centre. They just manage to hurt the algerian bodyguard, but killed one by-stander, and one 7-year old child.
And about street gangs, theres Latin Kings and Ñetas in Madrid and Barcelona. Mostly they are young latinos inmigrants, and are respected and feared, although their number is small. The Latin King in Madrid it´s a boy of 17 years old..., and as I know, there has been at least one gang killing in Madrid (some Latin Kings stabbed a 16 year old colombian boy).
No eres nadie hasta que alguien te mata.