"An extreme Islamic state that seeks to destroy Israel and whose leaders' hands are soiled with Jewish blood ... is emerging," the former prime minister said.
Netanyahu dismissed the view that governing might make Hamas more moderate, comparing the Islamist group to Afghanistan's Taliban and the religious hard-liners in Iran.
While many leaders in politics and the press echoed Netanyahu's harder line, some advocated restraint. Shimon Peres urged Hamas "to form a coalition with reality."
Avi Dichter, a former head of the Shabak Security Service, also suggested that Hamas be given a chance to prove itself.
"We should all remember they were not born with a desire to kill Israelis," Dichter wrote.
Dichter said that Hamas militants became killers because it was popular and then shifted to an electoral strategy because Israel's targeted assassinations made violence too expensive.