Vittorio Arrigoni, 36, was killed Thursday after being kidnapped in an apparent bid to free the jailed leader of Tawheed and Jihad, Sheik Abu Walid-al-Maqdas, The Guardian reported. The group has denied responsibility.
A video showed Arrigoni, blindfolded and bruised, his head held up by the hair. A text read: "The Italian hostage entered our land only to spread corruption."
At a rally Friday, Hamas leader Mahmoud Zahar said foreign activists were "our friends" and vowed the perpetrators would be punished.
He also suggested Israel arranged the killing to deter foreigners planning to sail to Gaza to break the blockade.
"Such an awful crime cannot take place without arrangements between all the parties concerned to keep the blockade imposed on Gaza," Zahar said.
Mkhamir Abusada, a professor at An-Najah University, said of the extremists: "They are dissatisfied with Hamas's failure to fight Israel and Islamize Gaza. ... The killing of Arrigoni attacks the credibility of Hamas as the source of stability and law and order of which they are so proud. Hamas will respond very harshly to ensure there is no repeat."