Dainotti had served a lengthy prison term for murder, drug trafficking and robbery but was released from jail in 2014.
"When people say the mafia is finished or has been defeated, sooner or later something happens that shows that the mafia is still here," said Francesco Lo Voi, a prosecutor in Palermo.
Trump and the leaders of Britain, France, Germany, Italy, Japan and Canada will travel to the picturesque resort town of Taormina on Friday night NZT for the two-day Group of 7 summit.
Taormina is at the opposite end of Sicily from Palermo and there is almost zero risk of there being any risk to the leaders and their delegations. The town, which sits on a cliff overlooking the sea, will be in lockdown and protected by around 10,000 soldiers and police.
But the execution of a mafia kingpin on a public street is hardly the image that Sicily wants to project to the world as it prepares to host the high-profile summit, which Trump is attending as part of his first international trip.