Fabrizio Piscitelli, known as 'Diabolik', was ambushed at Acqueducs, not far from Cinecitta, in the suburbs of Rome.
Media reports claimed that Piscitelli had recently been the subject of a drugs investigation and that his murder was linked to rivals keen to settle scores.
His attacker reportedly approached him from behind and shot him in the left side of his head.
'Diabolik', and his group of Ultras of the Northern Curva of the Olympic Stadium, was never far from controversy.
In August 2018, the Lazio Ultras banned women from sitting in the front rows of the stadium.
"La Curva Nord is a sacred place, women, wives or fiancées, are not allowed in the first 10 rows," they said in a communique.
"We invite those who choose the stadium as an alternative to a romantic day at Villa Borghese to instead choose another place."
Lazio Ultras have also been accused of anti-Semitism in their dealings with fans of rivals Roma who share the same stadium.