When police began investigating a double killing on the South Side last week, they realized one of the dead men was a reputed gang boss suspected in two horrific slayings more than a decade ago.Adalberto “Sabu” Santiago, a member of the Spanish Cobras, was found dead Friday afternoon with a bullet in his back inside a home in the 6100 block of South Justine Street in Englewood, according to police and court records.Another man, Cesar Cervantes, …